If there were aliens in Starfarer, I feel that they would be on par with the humans or weaker. The Domain pretty much crushed any alien threats, so in the absence of the Domain or any other large, organized opposition, some of them may have experienced a resurgence. However, my guess is that they would be much less common than humans. It's doubtful that their technology would be any more advanced than Tri-Tachyon's and more likely on par with the Hegemony's technology... or even lower. My guess is mid- to low-tech for the most part, but with high-tech in particular areas.
I can particularly imagine an alien race that invested heavily in phasing technology and weapons that can be fired while cloaked/phased, but due to specialization are weaker in most other ways. If their primary enemy was the Domain, which had an overwhelming military and vast resources at its disposal, then the only way to win would be to avoid a fight or use guerilla tactics; cloaking technology plays into that well. With the Domain out of the way, they may try to put a more significant foothold into the galaxy. However, their presence would probably be relatively small and their technology, with the exception of their specialty, would be more or less on par with the human factions'.
Just look at the following picture posted on the blog, at the ship between the Venture and the Falcon. When you look at that ship, tell me that doesn't scream "alien cruiser that cloaks."
As for their disposition, they probably wouldn't be particularly friendly, taking a real *** from humanity for a long time (decades, at least). And, smelling weakness, they might want to push for dominance. On the other hand, living under constant pressure, they may just be weary of fighting and want peace so they can just settle, expand, and grow as a civilization. Maybe both. Either way, they'd probably be wary of any humans, especially since humans (and pirates in particular) might still see the aliens as an enemy, or source of useful technology and territory to be claimed.