Actually, I think size of the heads will be the bigger problem. I have both the Red Star/FNG French and some Perry metal 8th Army and there is quite a disparity in size. The Perry WW2 figures really are titchy little buggers.
If you are after French in shorts you might want to consider the Crusader FFL. I converted a couple using Australian slouch hats and they look quite reasonable. A couple of problems though. Firstly, there's only one packet of FFL in shorts and not all of them are readily useable, one figure is wearing Bombay bloomers. Secondly they are all wearing scarves. This didn't worry me so much but after converting a couple I lost interest.
I do wish FNG would expand the range beyond Dien Bien Phu era paras.