I can't give you all the details you may want, but perhaps my own experience with them will be helpful to you:
My tank's a 55 gallon, with live rock. I purchased amphipods from Florida Aqua Farms and they were more than sufficient to keep my tiny octopus happy. They would breed like crazy and live in the live rock, and the octo would eat them like popcorn. With the octo gone, they're still all over the place, only more so. They do not seem to be pests or any sort of problem.
I would suggest that, if you have live rock, these guys are probably a nice addition to your tank's ecosystem, though i'm not sure how much they contribute to tank cleaning. Perhaps they do eat up detritus. If you have an octo and it's fairly small, they might make a good supplement to his diet.
rusty