As you say it just refers to a unit. I think that the description and the reference to being 3 to 6 ranks deep convinced me that infantry were intended, but it is not explicit.
If I were doing irregular cossacks in 28mm I think I might be tempted to base them individually so that they could be deployed in Skirmish order or closed up as Warband or Line. A bit like Light Cavalry work in Hail Caesar.
My cavalry tend to be in units of twelve. I don't fancy having to do units of 16 - 18 for cossacks. Options are bigger bases with fewer figures or fewer figures individually based so they can be spread out a bit. I think I prefer the latter. Of course BP is a game of units where frontages are the only important characteristic. Numbers of figures and depths are largely cosmetic.
BP's flexibility is one of its great strengths

