i have kept a clown trigger in a 55g before for close to a year or so with great success had him with a picasso trigger and a yellow tang
the clown trigger eventually grew a tad large and i sold him
the tang lived for a long time
picasso i kept cause he was a good size and not too big
do some research into aggressive fish and see what their max size is in captivity then make a decision
2-3 should be fine in a 55g ...also take into account the waste production they will put out AND when they feed they are messy so unless you have a very good skimmer don't bother i learned my lesson a while back and had nitrate problems eventually gave up for 2 years and now i got back into the hobby and have non-aggressive fish