Hey, those look pretty cool. I'll try and answer these questions for you.
1. I am not sure what the polygon limit is, but its very very high. Those models should work just fine.
2. Yes, it is entirely possible to import custom models with different hands or screen positions.
3. The minigun's barrel is animated in code. You can simply remove that code in your weapon mod for the M60
4. Textures for Turok simply need to be .PNG
5. Turok does not use skinned meshes like Half-Life with a skeleton. Instead, models are animated in pieces/segments like an action figure. This should not be a big issue for animating weapons, however. But it would make more complex animations like reloading and bending fingers require more planning.
All in all, everything you want to do is absolutely possible and fairly easy. Make sure you get Blender 2.8+ and the plugins that Behemoth Programmer created
https://behemothprogrammer.github.io/TurokBlender/index.html