In M&S, there are two types of aliens, Methane breathers and Oxygen breathers. Methane breathers are bizarre, barely comprehensible beings who can only communicate with oxy breathers in a limited way. They will be necessarily limited to NPC status, and in fact should be treated more as natural forces, like weather.
Oxy breathers, OTOH, are much more knowable. All are bipedal, and are in general perfectly playable, so these can, and should, be PCs. In designing the species design process, I have settled on a non-random stackable package method. You pick a bunch of packages, and add together anything those templates give you.
For example, there are six packages each for Diet, Body type, and Social Organization. Each gives an assortment of Attribute bonuses, innate skills, and Edges.
I'm also adding something to the Edge concept. Negative Edges. Edges are conditions under which the character has a bonus - for example a nocturnal character might have the Edge of "Night". Negative Edges are the reverse, conditions under which the character has a penalty - for example that same nocturnal character would have the Negative Edge of "Day". Edges and Negative Edges of course only work if the condition could make any difference. Night or Day don't matter if you are under artificial illumination. :D
Anyway, Negative Edges, and maybe this method of Alien species construction, will be folded into StarCluster 4 for sure!