I think there might be a big difference between what the two different Nassarius species can do. The Tongans are so big and strong I could see them (in theory) wrestle down another critter. The Caribbeans: not really.
I don't think it's necessary to have the Tongans. Even a bunch of small ones can manage a lot of leftover food without really having the individual strength of the Tongans.
I'd say if the Nassarius concern you just go with the Caribbeans