I think I could see the sense in having
one alien species as closely related as the Kzinti, just so there is an opportunity for human(-ish, including transhumans) PCs and alien
but humanoid PCs to interact on
some level, but beyond that I think I'd want to save PC aliens for the truly adventurous. If someone wants to be something like a puppeteer or a kdatlyno, he has to go a bit farther in learning how to think like an alien. But even those,
if their air, foods and worlds aren't mutually poisonous, are likely to be of similar ages (within a million years or so) and be competing for the same kinds of worlds in the same area of space. In other words, they probably have a common ancestor at the prokaryotic level.
If you think about it, allowing all your "don't know how to roleplay, but don't want to be a human" types to be Kzinti is a natural fit, like dwarven fighters in fantasy games.
Past that, PC aliens are rather pointless. It's the same problem as the centaur or dragon PC taken to the extreme. If the PCs don't share a common default environment, they're not going to interact well on the day to day party level. Sure, you can be a member of a species that lives in the heart of O type stars.... but what's the chance of any interaction with the rest of the party? Zero. Assuming you could make an Outsider or starseed PC, what would be the point in
playing it as a PC for more than a few minutes of game time every decade or so (Outsider)? Or at all (starseed)?
