The Warlord Russians certainly matched up in terms of size and proportions with the Perry set so I assume the French would match them too. But as far as I know the Warlord French set has opted for the pre-1812 uniform and has them in gaiters. The Perry set is in the post-1812 Bardin regulation uniform. The Perry set is also kitted out in campaign dress and thus has no gaiters. You could use Victrix French though. I use the extra Perry heads in the box and put them on Victrix bodies to add variety to my skirmishers (you need to cut the collars off the figures though and use a bit of green stuff). In the main I tend to have my Perry, Victrix and Warlord figures in separate units...but that's just my preference.
The Perrys will release some pre 1812 French very soon. Metals, I don't know if they will do plastics as well but if so it'd be rather far into the next year. I hope they will mix.. I really like the Warlord 3ups a lot and am looking very much forward to painting them.