not a severum, they get around 8 inches.
IMO for cichlids and smaller tanks, don't bother with Central American cichlids or South american(unless they are dwarf cichlids or keyhole cichlids)
You could get some african cichlids(smaller ones), there are some nice looking ones too! you could get some Yellow Labidochromis, they are cool looking. I wouldn't go with the bigger ones(Venustus, Electric Blue, Afra, etc. etc.)
Now if you were to go with smaller(dwarf) south american cichlids then you have good options aswell, and they upside is these guys don't rearrange the tank
you have some options, you could get Kribs, Apistos(lots of varities), Rams, Keyhole Cichlids, Goldeneye Dwarf Cichlids, i'm sure their are others..........
I wouldn't reccomend putting them all together, i'm just saying what I think are small enough for a 30 gal