well, as they get bigger they may (or probably will) get very aggressive and territorial. so, choosing tankmates can be very difficult. plus, the increasing salinity level would make it difficult finding something that would would not outgrow a 55 and be able to handle the increasing salinity. if you want something just as a dither, you could try some mollies. they could handle the increasing salinity but there's a really good chance they may get eaten. if they can survive long enough to start breeding though maybe the puffs will only eat the fry and not the parents. then you'd have a steady food source for them as well.
if I was going to try anything with them it would probably be mollies. 