I put a small powerhead in my mantis' 30gal tank. Just to keep the flow going and from detritus build up. Oh and I forgot to tell you, but if you haven't already bought your heater, make sure you get an unbreakable one! When the mantis gets bigger he'll be wanting to hit everything. My mantis is almost 4 inches now, and has never gone after a fish. The only time I saw him go for one was during feeding, when the fish I had in there tried to take his food that was on the stick.
I got rid of the fish in there just because I didn't want it, so now I'm thinking about getting one to put in there, just not sure what kind.
So for your question, only think to watch out for is the clowns maybe being aggressive towards the mantis, since hes small that wouldn't turn out good. Unless the clowns are small too. But you're right, they will do great eating the scraps if your snails/hermits dont. I'd go ahead and put a powerhead in there even if you didn't have the fish. Not a huge one, just something to move the water