Funny that every Star Trek alien was human except for a few minor cosmetic differences. Bajorans especially; just a little nose bridge wrinkle and presto, an alien. Aliens in Star Trek could all interbreed as well, which is not only impossible for different species but doubly so for species that originated on separate planets. The “Great Bird of the Galaxy” was apparently not so well informed.
Relative to interbreeding of the sub-species on Star Trek, the underlying premise is that all those sub-species had a common origin that is unknown, i.e. an advanced civilization that spread through the universe and then vanished but leaving their genetic heritage in the universe. Thus is the ability for them to interbred.