The older Foundry figures scale up ok with the Perry, because they sculpted them, although you may find them slighter and possibly shorter in some instances. I use them together, but then I like a bit of variety in my stuff.
I haven't had any of the new Foundry WOTR figures in hand because they look like monkey armed fugsters on the website.
Front Rank are about 1/4 bigger than the Perry stuff, and do not mix well unless you want a specific character figure to stand out from the rank and file.
Old Glory are much the same size as the older Foundry sculpts, but very much cruder, and aside from the rather useful artillery crew I have binned all mine in favour of the Perrys. Also, the usual caveats apply about inconsistent moulding quality.
Haven't had any Essex or Crusader so can't comment on them, but the Crusader look like nicer figures to me if that carries any weight.
For what it's worth, I'd stick to Perry all the way, and bulk them out with (old) Foundry if I had to.
EDIT: Have a look on here for how I've used some together recently:
http://leadadventureforum.com/index.php?topic=31935.msg377859#new