WhiffWhaf,
Westphalia is quite correct about the heads on the French Victrix and Perry models - however the Perry And Victrix British heads both plug into the collar. I'd suggest the bodies mix quite well (Perry British have separate arms) with arms of either maker. So a Perry body with Victrix arms works fine, and vice versa. The main problem is the Trotter packs - the Perry packs look correct and the Victrix are too narrow, leaving you with a choice of accepting them as they are, or cutting them in half and putting a shim in the middle.
As for the heads themselves - the Perries are, as Captain Blood says, a bit more natural and the Victrix are a little cartoony - both styles are very nice but a bit different. It might be that you use one set of heads for one battalion and the other for another.
As for the French infantry - well the Perry sculpts only allow you to swap heads after some surgery with a scalpel. And I think the Perry French faces are "soft" compared to the British (or indeed the WotR plastics). The packs on the Perry and Victrix French swap very happily with one another.
Sorry, I can't really comment on the cavalry.
Have a look back through this nutter's blog and you'll see examples of mix and match:
http://britisharmywaterloo.blogspot.co.uk/2011/09/serjeants.htmlRegards
Malc