I use them toghether, but then I have a mix of Foundry, Artizan, BAM, BTD, Crusader and a few others, and if you have lots of variation you can get away with it. But if you just have Foundry and Crusader, you'll have small guys and big bulky ones, which might not work so well.
I find BAM and Foundry are close enough in size. Most of BAM are late war though, although there are some early war packs (like Foundry). The later Artizan releases are second best in terms of size (though again, they are late war).