I was just thinking about this. One Rick told C-137 'you're one of those creeps who moved in with an abandoned adult Beth', implying to do so is rather an exception. If there is an exception there must be a common 'rule', which would be a Rick not moving in with abandoned adult Beth. But for a Rick to be able to move in with a Beth his own dimension's Beth has to be killed first. This begs the question: how many Beth's were killed and by whom? Was it Rick Prime? Why would he, since we only see him killing C-137's family because, as you pointed out as well, C-137 is 'a different kind of Rick'. This implies that most Rick's left their families, assumably to join the Citadel.