We need to know what kind of shiet and algae you're refering to in order to recommend an effective clean up crew. Otherwise, turbo snails and hermit crabs make up the generic clean-up crews
I like nassarius snails, ceriths, astreas (turbos). I'm personally not a big fan of hermits. Although I still have some from my original purchase a couple of years ago.
depends on how "dirty" your tank is. Buy a couple and see how they do in your tank and after a week, if it needs more go buy more. Don't get too much at one time. For your 55, I would say start off with 5-7 snails and 3-5 hermits and see if thats enough, then get more if need be. I know that they seem few but you have to guage how effective each species is and then go out and get more of what seems to work.