Consider, however, that blood is but her dietary requirement. She still needs Essence. It only drops one point per month so long as she doesn't tax her powers, but still, at some point she needs a live meal.
But, yes, they sound like the good guys. APBs rap sheet is probably her foiling some corrupt official/executive nefarious plan and paying the price for doing the Right Thing®.
Ace may or may not have worked for Ares - he may have been undercover there but working for another party. And despite what Ace says, we haven't seen any evidence that his loyalties changed prior to his amnesia. He came to Aspen looking for someone - an unknown female operative, possibly Ares but possibly for whatever third party he may have worked for. Not Katsina, since Katsina was sent to look for him, or at least someone that looked a lot like him - there may be a whole squad of Ace clones for all we know. So maybe APB, but like Ace points out IC, it doesn't really fit, but it can't necessarily be ruled out. To my knowledge, he has not shared the dossier that was attached to the message he shared with the group, so we don't have much information to go on.
We don't know how recently Katsina became a vampire. It may have happened after coming to Aspen for all we know. Likely not relevant.