Any tank less than 4' wide simply doesn't have enough floorspace for more than one species of substrate spawner. Multi's are colonial fish, something like the brichardi complex species or even Julies, where the older fry will help defend the smaller ones. Eventually the maturing young males will want their own territories while the females establish their own pecking order, but the entire bottom of the tank will be taken over sooner or later and you'll need to move some of them out.