I believe that whatever happened to Randy and the techie group at the water works happened before Michael and the tower group got there. They remarked on how fresh the blood was, and given that every other hideout that CJ was responsible for is heavily fortified, I don't believe she would have left a group at the waterworks full time with no added defenses. As for Randy the zombie being Randy from CJ's group, it's pretty well confirmed I believe by the fact the he was waiting for Glen and Pete outside of the safehouse that CJ had put them in. Being a member of CJ's group beforehand he would have known where the safehouses were, and it would have been simple elimination to find the one that was occupied and stake it out.