i think one oscar and 2 severums would be fine in a 125 gallon with proper filtration and water changes. the rams would be very iffy due to the fact the oscar may decide to snack on them one day. 2 female convicts would probably be fine with the mix as well but personally I think I'd stick with the oscar and severums and maybe a few non-cichlid fish. perhaps a school of silver dollars or some pictus catfish.