You know what I think? I think you people think about this
way too much and have a lot of free time on your hands.
Pick a species and play. Pick two species, even. Read if they're able to crossbreed or not (human/human, human/near-human, human/humanoid, near-human/humanoid, etc; the Wookiee usually indicates if they can or can't breed with x or y type) Take the most prominent ability, throw out or nerf the rest, and move on. This is a roleplay, not a biology course. xD
PS: Like all theoretical science we can't prove or disprove FTL and all the sort because we have no way of personally testing or observing it (gee, wonder why) so it's subjective to how much we actually know already. The same could be said about species that don't actually exist in real life, or species that might indeed exist in real life and for some ridiculous reason have a compatible quantity and structure of genetics as we do.
![Er... what? o_O o_O](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)