Yes get your two fixed right away (the effects don't always 'take' for a month or so). It's important to decide ahead of time which room will be a 'safe' place for whose cats (and safe for the others by having the closed door at different times). None should feel from the start that they have the run of the place and the other(s) don't. It can take a couple of weeks before they even tolerate the other cats' presence, let alone not start fights. While female cats can try to dominate males by hissing and scratching, they're often lighter and won't do well in real fights, plus two-to-one will make it hard on your friend's cat if your guys decide to get serious. Certainly I would not leave them alone together until you're very sure they really get along ok at any time, and until all feel very much at home there, with at least a couple of 'known' high places to run to where other cats can't reach them (only room for one cat to land and not be accessed from anywhere else). It may be best to establish specific feeding times from the beginning (or ahead of time if you now free-feed) with plenty of 'personal' space for each, and one of you two monitoring things, e.g. keeping others away while each finishes up, and putting bowls away til next time, or twenty mins. later if they can't finish at once - that's how I do it here. Be sure to have at least two, if not three litter boxes (not near food) and a couple of waters, and, ahead of time, inspect the house very carefully to be sure there are no e.g. difficult hidey holes in the basement, or unexpected access to outside. It might be be best to leave all at their own homes til the entire move is made and some familiar furniture's placed, especially in 'their' rooms, then bring them over. They should all get a chance to roam and smell neutral territory on their own for 1++ hours every day, and only gradually be introduced (after lots of under-door sniffing) after 1-2 weeks. You'll also have to gauge for yourselves how they react to each other when the time comes, as you can't predict about a given cat. I have two young male cats and one 15+ female, but she was here first, she's a Maine Coon, believes she's a princess and is very scathing of them, but I also know if at least one of the males were to decide to really attack her, she likely would lose (he's strong!). They respect her though, and she has where to go if they do get nasty (though haven't so far). She has no interest in them apart from hissing them off if they get in her face.