Those are nice. Yes get a qt!!!!! Your tank is small by sw standards so your possibility of livestock loss is minimal. However once you go big the amount spent on filling those tanks isn't worth risking by not qt.
Now remember once you add the clowns your done!!! No matter what else you find. Give it a good month and then you could add again, fish wise. If the fish are good after a couple weeks you can add a few shrimp, or crabs, ect
If u have some alage on the glass try a few snails and see how they do. If all is good after a wk or 2. Add your clown"s". If you want 2 add them together tho
They say the shell is hard enough. Just buy 1 and see. It should pick at it. You want raw tho. Also my saddle would eat live ghost shrimp meant for my lion