I would wait and see how he is for a few weeks before worrying about leaving him alone with the other dogs. Watch them interact, see how they do or do not get along, who "tests" who ... hopefully you have a bit of time before you go away and need her to watch the dogs ... it could be this is a perfectly good dog who just didn't get enough training and ended up too much for the person to handle - being they are such energetic dogs, that's a s likely a scenario as any as to why he became a stray! And if he is neutered - which I am guessing he is, you run less chance of him developing aggression issues later.