Probably only going to be able to do a single species tank with a 90l, maybe two at a push if you stuck to no more than 2-3 specimens of each species.
As a species tank many substrate/rock dwelling Tang cichlids would work well in a 90l, shelldwellers (brevis, occelatus, ornatipinnis, multies), the smaller julies such as transcriptus or ornatus, altolamps (calvus, sumbu "shell" comps). A pair of goby cichlids would work well too but bear in mind they need a veggie diet. Would avoid the open water swimmers such as cyps, paracyps etc as they need a really long tank.
I had two pairs of brevis shellies in a similar sized tank, was a great little tank to watch, and they bred during the time they were in there (free fish are always a good thing!). You have to bear in mind virtually all Tang cichlids are very territorial and all need their own space in order to display natural behavior. Males and females can be aggressive (one of my female brevis spends more time shooing away trespassers than the male she is paired with!) so it's not just the males you have to worry about.
