Editable mesh, of course that doesn't stop you from working in editable poly and then just adding an editable mesh modifier on top of it and collapsing to it, it's what i do. As for max polygons, i don't know if there's a "max" that the game accepts, the chaos lord i mentioned has the highest polycount(12,600) in-game as far as i'm aware. Either way, i wouldn't go past 15,000 polys for "unique" units like commanders, heroes, super units or whatever, but that's just me, Horus would tell you to keep it as low poly as you can, it's up to you.