I share the same concern with this question, and I will add another question that is of relevance. If a mounted character, presumably on a steed with a 25x50 mm base, were to join a unit, would it be ok to make two of said infantry to place behind him on 25x25mm bases? This way, with another twenty figures on the normal 20x20mm bases, will be able to rank up nicely around him.
THis is what I mean:
M = Mounted character
I = Infantry on a 25x25 base
B = Infantry within said unit
M/I = Mounted and 25x25 Figure
B B M B B
B B M B B
B B M/I B B
B B I B B
B B I B B