Go with the traditional and stay away from urchins its too smal for them. If you stick with snail your good, but if you mix snails and hermiths; here what happens.....
Hermiths are like a gang, they work the snails and kill them one at a time while still doing their job until no snails are left. Then as they grow well they also want each others shell and battle over them like teenagers and kill each others until the bigest remains. New hobbyist don't notice this until the algaes are evrywhere and they don't know why but see all these empty shells laying around and think the snails died of natural causes , so they buy new snaisl and the biggest hermith get them again..
So chose the snails and resist the temptation to put hermits and crabs. OHHHH they'll tell you this one is reef safe, they're opportunist . 10 The trochus are the best for algaes, 8 t turbo or tiger Nassarius or 6 Babylonian snails for detritus, 1-2 brittle starfish for detritus as well, the night crew lol , you don't need a cucumber in your tank unless you put a lot of herbivores fishes that make a lot of waste. Stay away from the turbo mexican snails , curiously these snails are for cooler waters 79oF where most tanks are at 80 + you basically kill them slowly so they don't last very long. Plus they eat a lot and starve very quickly if their's no food. And a pain to turn over all the time..... cerith are excellent for your tank size if you go for 20 you'll have enough . One thing to do is when you clean your glass always leave one undone so they can have food. If your rock are filty clean all your glass panel so they can concentrate on your rocks and sand. For your sand a fighting or strawberry conch. Try one see if he can do the whole tank by himself. if he leaves a few patches untouch thats ood that means he has enough food , don't add another one. other wise you'll have to supplement with dry seeweeds. Its also depends what you have in your tank as rockwork, if you have a backwall lots of variables plays into this. One thing your fish can survive with a gravity of 1.021, but not your inverts, its 1.023, better keep your gravity at 1.024 even 1.025 to make sure.
