Orwell's unit was almost all anarchists.

Also, Orwell served in POUM's militia, not with the anarchists.

The unit's members, not its flag, hence "almost all", which would make no sense describing the unit's affiliation.

It is necessary to explain that when one speaks of the P.S.U.C. 'line' one really means the Communist Party 'line'. The P.S.U.C. (Partido Socialista Unificado de Cataluña) was the Socialist Party of Catalonia; it had been formed at the beginning of the war by the fusion of various Marxist parties, including the Catalan Communist Party, but it was now entirely under Communist control and was affiliated to the Third International...Roughly speaking, the P.S.U.C. was the political organ of the U.G.T. (Union General de Trabajadores), the Socialist trade unions.

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In any case the loose term 'Anarchists' is used to cover a multitude of people of very varying opinions. The huge block of unions making up the C.N.T. (Confederacion Nacional de Trabajadores), with round about two million members in all, had for its political organ the F.A.I. (Federacion Anarquista Iberica), an actual Anarchist organization.

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The P.O.U.M. militiamen were mostly C.N.T. members, but the actual party-members generally belonged to the U.G.T.

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In Barcelona there had been a series of more or less unofficial brawls in the working-class suburbs. C.N.T. and U.G.T. members had been murdering one another for some time past; on several occasions the murders were followed by huge, provocative funerals which were quite deliberately intended to stir up political hatred.

Those are from Homage to Catalonia.

I include the anarchists (CNT) and the Catalonian independent Marxists (POUM) among the "small-c communists." We can quibble about this designation, but I think it's fair...I'm also sure that members of POUM would not have had any problem with this label, being self-proclaimed orthodox Leninists.

The minority U.G.T. Leninists wouldn't, but the Catalan draftees who were members of anarchist unions (which were strongest in Catalonia) would.

it is indisputable that the parties for which Orwell fought were...violently intolerant of any opposition

If they were so violent, they wouldn't have let the Communist minority grow in power until they killed them. They were really violently intolerant of some opposition, which is not the same quality of thing, for many are violently intolerant of some opposition, the extreme stances being violent intolerance to no opposition or all opposition.