Its usually quite random who they "team" up with, could be a random townie or a premade I like. I just go with whoever my sims seem to have chemistry with. I'll have them talk to and hang out with several "canditates" and then choose one that my sim is attracted to the most.
One exception is the legacy I'm doing now, which is my very first. It started with a fresh CAS sim, and she had to marry whoever brought the Welcome Wagon fruitcake after she moved in (Big thanks to Sindocat for the idea!). It was messy, because the random generator chose Strangerville as her place to live. The visitors were of course all infected, and my sim couldn't really talk to any of them :D Currently on Gen3, working on Gen4 :)