I'm a big fan of only snails. I have probably about... 15 large size astrea snails in my 120, maybe 2 or 3 nassarius, and I had a conch that started off 3" long, and then died in the last 3 months at some point, but his shell was 6-7" long when he died.
No hermits, no emeralds. I've always had bad luck with them, because I always saw them killing astreas for their shells as was mentioned by Elan.
I'd almost be willing to try blueleg hermits again, but that's about the extent of crabs I'd be willing to try. I don't think you'll have too much of a problem of predation if you got one of those GIANT clean up crew kits from one of the online places where they just include hundreds of cleaners. I've never done that because of the price, and only have pieced my crew together when I had a couple of bucks here or there.
Oh, and I also have 2 tangs and 2 dwarf angels who all eat algae. My sailfin eats anything green, and my Tomini tang eats a lot of the red/brown crud that grows on the rocks, as well as green stuff.