I second the previous post. You cant go wrong with ReefCleaners and the tank size feature on the website will get you what you need. I prefer more snails than crabs, but some people like the scurry of the crabs around the tank.
I usually stick with Nass, Astrea, Cerith and blue leg hermits. You actually don't need a ton to maintain a 120. Also, depending on your definition of cleaner, you might also consider sand sifting gobies (if you have sand) etc..
Definately check out the reefcleaners packages to get an idea of what creatures to get! John is great to work with and will build a packages just for you. But I wouldn't necessarily recommend adding 120ish creatures all at once like some of his packages include (unless your tank is very established)