What are the dimensions of the tank? And it Depends what you mean by larger fish? A 30 gallon tank is too small long term for even a single red tail shark. They need tanks of 55 gallons or more as they get very terratorial as they get older. It's also said to be better and is advised to only keep one red tail shark per tank as they will eventually turn on each other. More than one could work if the tank was huge.... As in 125 gallons+
I'd say in a typical 30 gallon tank you'd be looking at 1 or 2 fish at around 3-4 inches (adult size) like dwarf cichlids, dwarf gourami... Maybe bolivian rams (as your centrepiece fish and then maybe a shoal of around 10 smaller fish and a group of smaller corys for the bottom (if you have sand)