you need to get alot more to your cleaner crew or your going to lose in a never ending battle with algae.
Oh yeah, the fish question.
Most triggers and puffers will eat snails and/or crabs. Triggers may even eat smaller fish, like your damsels and clowns. If you want to add more fish, I'd suggest sticking with one of the reef safe-ish triggers (they have the potential to eat those animals, but are less likely to than other species) such as niger, blue cheen or pink tail (I have a pink tail in my reef tank).
My pink tail trigger is an angel. He doesn't bother anyone. And they are cute!!!
Any trigger will eventually outgrow your tank, but I think a 90 would be okay for a small one for a while, at least.