OK, I'd like to talk about Alien species in SFRPGs including ones you might borrow from other sources and stick directly into a game you're creating.
For a little known pair of entries, icd like to bring up the Spectrals and the Dragoncrests from the Living Steel RPG. You can look them both up on the net but in short the spectrals are implacable bad guys who must feed on sentient races to live and have some unique methods of invading a world in a way that lets them maximize the chances of each spectral feed directly on a sentient being. They kinda border on mystic in some ways but have a hard sf feel to them in some.
http://living-steel.wikia.com/wiki/Spectrals
Dragoncrests are a less hostile alien species and have some interesting views of philosophies that might make them an interesting addition to a sf campaign.
http://living-steel.wikia.com/wiki/Dragoncrests
Moving onto a RPG alien more people might have heard of, there are the kafer from 2300. These guys are very alien physically and mentally, possessing volatile intelligence that goes up when the kafer experiences fear, pain, danger, hunger, etc. Think of something like the human adrenal gland except it briefly boosts brain power intead of physical abilities. Here's a review of their sourcebook. https://www.rpg.net/reviews/archive/15/15235.phtml
Traveller's hivers are an interesting species that evolved from a non predatory, non carnivorous species that have a fundamentally and radically different mentality than many species, but a logical, plausible one nonetheless. They eschew and even abhor violence and are essentially natural socialists who prize cooperation and responsible manipulation for the common good. Look up traveller hivers if you're unfamiliar with them.