Well, it depends on what size tank. There are a couple "standards" that most people get, such as some type of shrimp (I have 3 different types, but you should probably go with either peppermint or cleaner), several different types of snails (again, I have some Astrea, some Margarita, some Cerith, some Nassarius), and maybe some hermit crabs or some other type of invertebrate algrea/detritus eater. If you go by the 1 cleaner per gallon, that'll be a good place to start, though some have many many more, and some have quite a few less.
As far as flow, get powerheads that will give you about 10x - 15x turnover. Turnover is calculated by the gallons per hour divided by the tank size. So, if you have a 55 gallon, and want 15 x turnover, you would want powerheads that would give you a total desired flow around 825. It's also best to split that up with at least 2 powerheads, one on each end, so you can have the flow more circulating, rather than hitting one general spot and spreading. To do this, you would face the powerheads at each other and turn the flow valve to go towards opposite corners.