So far, I've mostly assumed populism to be an amalgation of predominant right-wing nationalistic ideologies which represent "the peoples". But recently when I was reading some articles about the EU elections, I found that some authors also add some far-left parties to the populist definition. The PopuList for example consists of European parties that can be classified as far right, far left and/or Eurosceptic.

Here what the Wikipedia says about left-wing populism:

Left-wing populism is a political ideology that combines left-wing politics and populist rhetoric and themes. The rhetoric of left-wing populism often consists of anti-elitist sentiments, opposition to the Establishment and speaking for the "common people". The important themes for left-wing populists usually include anti-capitalism, social justice, pacifism and anti-globalization, whereas class society ideology or socialist theory is not as important as it is to traditional left-wing parties. The criticism of capitalism and globalization is linked to anti-militarism, which has increased in the left populist movements as a result of unpopular United States military operations, especially those in the Middle East. It is considered that the populist left does not exclude others horizontally and relies on egalitarian ideals. Some scholars point out nationalist left-wing populist movements as well, a feature exhibited by Kemalism in Turkey for instance or the Bolivarian Revolution in Venezuela. For left-wing populist parties supportive of minority rights among others, the term "inclusionary populism" has been used.
Some left-wing populist parties from Germanic areas:

Austria – JETZT – Pilz List, Communist Party of Austria

Denmark – Red-Green Alliance, Inuit Ataqatigiit, Socialist People's Party, Republic

Germany - The Party of Democratic Socialism was explicitly studied under left-wing populism, especially by German academics. The party was formed after the reunification of Germany and it was similar to right-wing populists in that it relied on anti-elitism and media attention provided by a charismatic leadership. The party competed for the same voter base with the right-wing populists to some extent, although it relied on a more serious platform in Eastern Germany. This was limited by anti-immigration sentiments preferred by some voters, although the lines were for example crossed by Oskar Lafontaine, who used a term previously associated with the Nazi Party, Fremdarbeiter ("foreign workers"), in his election campaign in 2005. The PDS merged into the Left Party in 2007. Die Linke is considered to be left-wing populist by some researchers.

Luxembourg – The Left, a democratic socialist political party.

Netherlands - The Socialist Party has run a left-wing populist platform after dropping its communist course in 1991. Although some have pointed out that the party has become less populist over the years, it still includes anti-elitism in its recent election manifestos. It opposes what it sees as the European superstate.

Norway – The Socialist People's Party was a splinter group of the Norwegian Labour Party (DNA) founded in 1961. SF was principally dissatisfied with the pro-NATO/European Economic Community external policies of DNA. A group centered on the magazine Orientering had been expelled from DNA. The party merged into the Socialist Left Party in 1976.

Sweden – Left Party. The party originated as a split from the Swedish Social Democratic Party in 1917, as the Swedish Social Democratic Left Party, and became the Communist Party of Sweden in 1921. In 1967, the party was renamed Left Party - the Communists; it adopted its current name in 1990.

United Kingdom - Labour Party leadership of Jeremy Corbyn. The Scottish Socialist Party is a left-wing political party campaigning for the establishment of an independent socialist Scotland.

United States - Huey Long, the fiery Great Depression-era Governor-turned-Senator of Louisiana, was an early example of left wing populism in the United States, advocating for wealth redistribution under his Share our Wealth plan. 2016 presidential candidate and Senator Bernie Sanders has been described as a populist. Senator Elizabeth Warren has been described as a populist.
Some notes from an article:

The rise of both kinds of populism is the result of the long-term failure of neo-liberal policies, as many already know, but it is also "a necessary dimension of democratic politics" as the political philosopher Chantal Mouffe explains. (...)

As a consequence of framed democracies, populism has become the only productive form to take into account the demands of the people and to promote collective participation. According to Mouffe and her fellow political philosopher Ernesto Laclau, whose investigations of populism have now become central among political scientists, if democracy wants to preserve its superiority among other political systems, it must return to the people.

And this is what populism does. It is "a way of constructing the political on the basis of interpellating the underdog to mobilise against the existing status quo". It brings together different demands in opposition to a common enemy. Laclau and Mouffe do not consider populism an ideology but rather a political form capable of articulating identities, interests, and needs that have been delegitimised by centre-right and centre-left parties.

Although both apply the same principle - bringing together a crowd around a political idea in order to shape an "us" against a "them" - the concepts used to define these groups are radically different. This is also evident in the emotions each side uses to mobilise voters: fear of the foreigner on the right and hope for a better future on the left. The former is rooted in hatred and indifference, and the latter in justice and equality. The right-wing populism of Donald Trump and Nigel Farage, expressed in their "Make America Great Again" and "Leave" campaigns, restricts the national identity of "the people", excluding immigrants, refugees, and any Other definable as "foreign" to a sentimental ideal. Although exclusion is also present in the left-wing populism of Bernie Sanders and Pablo Iglesias, they do not exclude categories of people but rather those sectors of the establishment in the service of neo-liberal global corporations.
The source:

To me, it sounds like another form of communism or proletarism in some ways. Is this an economic factor? It seems to me like many of the left-wing populist parties define the peoples as a social class vs. the right-wing concept of ethnic nation. There can maybe some exception with an ethno-national element, like the Scottish Socialist Party, which could be called socialist nationalist? So what do the ideologies have in common, what is populist about each spectrum, considering they are on extreme opposite ends? ("far-right vs. far-left") Would it be the opposition to the elites and neoliberalism? What about Euroscepticism? Some left-wing populist parties are Eurosceptic, which makes it a common theme with some right-wing populist parties, however other right-wing parties prefer to work within the EU.

Another article writes populists could become a bigger and more influential bloc if they gathered together as an anti-establishment alliance:

Matthijs Rooduijn, a political sociologist at the University of Amsterdam, said populists had won 218 of the parliament’s 751 seats. All kinds of populism were on the rise: left, right, as well as groups such as the Brexit party, which cannot be easily categorised.

These groups will remain divided in the European parliament, blunting their influence. But their greater numbers in Brussels and Strasbourg, combined with the deepening fragmentation of European politics, will make it harder for traditional parties to find a majority.

The rise of populism was driven not only by immigration, European integration and economic issues, Rooduijn said, but the narrative that an “evil elite” was in charge. “This evil elite can be a national elite, it can be Brussels elite or European elite.”

Many, but not all, populist parties are sceptical or hostile to the European project, he said. “Euroscepticism is framed in a populist way, so it is about Brussels technocrats, who do not listen to ordinary citizens.”

The European elections resulted in traditional centre-right and centre-left blocs losing their majority for the first time in the 40-year history of the directly elected parliament. Greens and liberals made striking gains, ending the cosy consensus of centre right and left.

While populists could continue to grow, their fortunes could also change quickly, Rooduijn said. “It depends on the topics that are salient in the media, it depends on the performance of the leader, all these things matter a great deal and it can really lead to big shifts in success or losses over very short periods of time.”

Non-populist politicians will dominate the next European parliament but will be forced to confront more opponents who paint them as an out-of-touch elite.

Populist MEPs will remain divided between different groups in the European parliament, which will blunt their ability to secure senior posts and set an agenda.

What is your view? At the moment, it's clear that right-wing parties by themselves don't have enough figures to infuence significant decisions in the EU. Could right and left-wing populists put aside their differences and form a bloc to oppose and block establishment? Is this realistic, when even similar right and left-wing populist organisations differ on some issues? And are left-wing populists preferable to center and center-right parties?