Thanks for the info on Libmod. I don't really understand the config stuff, but I have managed to get some I-44 units to work. The suggestion to addthe I-44 addons into the Libmod is working out well. I have just copy/pasted them to a Libmod backup folder.
All infantry so far has worked well. I had one crash due to a grenade but I missed what kind so it might happen again until I can remove it from the missions. I-44 bangalores etc. are compatable so far, as are their weapons. I havent seen any custom animations though.
The Berlin Mod Stalin tank has a white gun, I assume due to some textures being hidden.
I managed to get all tanks to drive around together, but combat needs to be tested. I don't think it will be pretty.
I'll update if anything interesting happens just in case someone else wonders the same thing later.