Given the tank dimensions(fairly flat) I agree that the multis will expand to fill the space
.Having said that I often have rainbows or blue eyes with my shell dwellers(they enjoy the same water).I keep Ps furcatus with multis, have M.eachamensis with my similis,G maculosis with my ornatapinnis,C.bleheri with my speciosus and Cairnsicthys with my brevis.Im able to collect eggs from the rainbow and the shellies seem to have no trouble protecting their young