I would stick with turbo snails, trochus snails and ceriths. Avoid the fabled 'detritus eaters' like nassarius and sand sifting stars. A Holothuria sp. would be a better choice for a sand cleaner, stocking one should not be taken lightly.
I have a standard 29 with a 20g refugium. I am stocked with 1 swisguard basslet, 1 blue/green chromis, 1 disco blenny, 1 6-line wrasse (i don't recommend). My inverts are 3 mexican turbos, 3 trochus, 1 royal urchin, 3 blue legged hermits.
With 2 clown in a 30 I cannot see you adding any other fish. Especially once clowns pair they may very well dominate your tank. There are plenty of other interesting fish for that tank. A pistol/goby pair would do nicely (yasha haze + red banded pistol), you could probably add a crypic wrasse and a fang blenny.
A toby puffer (Canthigaster leoparda) would be a good alternative stocking, assuming you don't mind not having inverts. Maybe throw in some other fish that enjoy low light like a swisguard basslet and/or cryptic wrasse/possum wrasse. Possibly even a 2 spot blenny to make up for your lack of CUC.
Whatever you choose, sump and skim your tank. You will thank me later.
Edit: I almost forgot. You can totally keep a Gymnothorax melatremus (dwarf moray) in that tank.