pick up one model inmport it to blender thru M2i then import another parts of another races and rigifi them but that work for 6 hours ++ if you want to have them perfect
Jump on YOutube and search for tutoriasl for blenmder rigifign etc .... your problems for now is not method or miss-undestand of some soft or bad code you need to learn with blender
The copy-paste tool isn't native from Blender and it doesn't work well as addons. Some polys usually dissapear or just morph weird. My advice is:Import a M2i from the race you want to use as "basis" (your final model will have all the animations).Behold how are its parts in segments. You must take the meshes from the second race you're looking to put in the first one, copy the mesh name of the first one, delete it and punt the mesh from the second one with the name of the first. Then drag and drop that mesh into the "armature" family of the first. this one isn't exactly a tutorial of what are you looking for, but it would be helpfull.
Don't worry mate, just remember to share with us some screenshots of your progress. I'm really curious about what race are you trying to create and I'll be glad to see the final result. Don't be shy, if you have other dubts, I'm PM friendly.