The Artizan Australians and Sikhs are fine but all their British stuff isn't.
The Foundry figures should be ok with the TAG figures, but I don't have any Foundry Chindits to be sure. The Perry Germans are a little snaller than the BTD Germans but still work alright, and the BTD figures work with TAG.
No Idea about Chinese, probably not though.
I've only seen WWII Ghurkas from TAG, there are Foundry Ghurkas but they're colonial era ones.
Again, I don't think so. You could probably use some Copplestone figures from his BoB range if you didn't care too much that their equipement is a bit early. He does Ghurkas too I see.
Battle Honours WWII probably cover the ranges but their figures are not always good and most of their ranges of different troops from a single nation are the same figures with different heads. Their figures are also quite small.