What I don't remember is how much light you have. If your small tank is like mine (>6 watts/gallon), you will get strong algae growth pretty soon. To give an example of one tank, here's what I have in the 20 as a cleaning crew:
11 Astrea snails
6 Ceriths
many strombus maculatus (they keep making more)
1 peppermint shrimp
~15 blue leg hermits
A few hitchhiking hermits
5 scarlet hermits
I guess that comes to 1 grazer and 1 scavenger/gallon. It is too much, probably, but there are historical reasons. Believe it or not, there used to be more. I have already evicted/traded in 10 blue legs, 1 red clinging crab (like an emerald), 7 keyhole limpets and 2 peppermint shrimp. [Insert rant about TBS having too many animals in its "package"]. Nonetheless, the tank has been kept quite clean by this crew for about 1.5 years.
I agree with Brian. Go for a few of each to start with. If it were my tank, I would avoid turbos as grazers because of their size, and go with the smaller species like ceriths or the little Strombus maculatus. Maybe a few astreas or margaritas. For scavenging, nassarius would be good, and/or maybe a few of the little bumblebee snails. If you need pictures, look
here. As far as hermits, I find them indispensible, and I'd suggest adding a few scarlets. You will also have a variety of algae to deal with, so diversity is important.
But that wasn't very clear. Here's a suggestion for a start:
Grazers: 6 ceriths, 2 margarita
Scavengers: 3 scarlet hermits, 3 nassarius.
You might also think about getting a detritivore package from
IPSF or
Inland Aquatics, to get some worms, stars, pods and other goodies.