I've pretty much found that if they don't like where they are and who their neighbors are they will move. Usually thowing off babys as they go. I can't think of any place in my tank that they have gone to war with each other, which of course doesn't mean that they won't, just that they didn't. I'd keep an eye on them, but I feel you should be fine.
You might want to tell the girlfriend about this site so she can research ahead of time before buying.