Are we allowed to add other game ideas into this to form a new mod like adding chars and tech from another game to expand endless space as long as we don't use any copyrighted material? If so what are the guidelines for doing such a thing? For example say i wanted to make a new char race and add a new tech tree to that char. Say make a race of living trees and add an organic evolution tree, would that be allowed?

If you want to avoid legal issues, you are always better to make original stuff, even if the ideas are borrowed from other games, movies and such. Some may not care that you are stealing/reusing ideas, but some others may not. I know some mere modders who have been pissed off when other people reused their ideas/work to make other mods. Intellectual properties are always a touchy subject...