I agree with Joe because of the size of the tank and the aggressive levels and sizes of the fish... shell dwellers would make the most sense.
For decor, lots of shells, gravel or crushed coral substrate, and plenty of rock structures through the entire tank.
The last tank I set up for shell dwellers was a 25 gallon, and I lined the back and sides with rock structure, building it up high all the way around, then left an open space in the middle for lots and lots of shells. I added some silk plants to the back corners and a few smaller ones among the shells in the middle. I ended up with a tank that was easy to clean, happy fish, and looked good all at the same time.