Water has very little good bacteria in it, don't bother using his old water. Just make sure you tank is stable( cycled). Ask him what his ph and kh is. Try to match both of those and you should be fine.
16 in a 55 is fine for most Tropheus. If they were wild I would re-think things but it doesn't sound like they are. Just watch for overly aggressive males as sometimes one fish can be too much for a 4 foot tank.
Ask him what he feeds and if at all possible get some food from him so you can slowly change them over to what you feed.
Keeping things stable and the water clean are the two most important factors with tropheus.
Good luck
I love mine 
16 in a 55 is fine for most Tropheus. If they were wild I would re-think things but it doesn't sound like they are. Just watch for overly aggressive males as sometimes one fish can be too much for a 4 foot tank.
Ask him what he feeds and if at all possible get some food from him so you can slowly change them over to what you feed.
Keeping things stable and the water clean are the two most important factors with tropheus.
Good luck