Will this mod work for an existing save?

Yes, this mod modifies the spawners' prototypes when the game loads, an existing save should work without any issues

Will this mod work with an other mod that modifies enemy spawns?

If you place it after that mod in the load order - probably yes. Depending of course on the other mod's implementation.

Will this mod work for base game (no space age)?

Yes. This mod was built with space age in mind, but using it for the base game will work just as well.