i would just put them in the 55g you have and then save up for a bigger tank, its kind of pointless to waste money on a 75g for them when they will outgrow it within a few months and for 2 Os, GT and Sev you are probably going to need at least a 6ft 125g in order for them to all have room and the water be able to stay clean between wc, aggression wise they should be fine IME my GTs were only reason aggressive when defending themselves as long as others left them alone they didnt really bother anyone other than the occassional chase here and there