I posted this question in a post on the Ancient Fish page, but I thought I'd post it here, too. My girlfriend has a 20g tank with a school of 7 young tiger barbs, 1 oto catfish, and three dwarf puffers. All of the fish get along great. We regularly get pest snails from the LFS to act as puffer food. I wouldn't want to put mystery snails in, as I fear the puffers would torture them to a slow, painful death. We had some ghost shrimp to serve as puffer food, and over time they've disappeared, too.
The Oto does a great job of taking care of the algae, but even doing weekly 25% water changes/gravel vacs doesn't seem to take care of the brine shrimp and bloodworm debris the puffers leave behind. The ghost shrimp used to do this job just fine. We were thinking about adding one or two of those small Red Crabs to the tank. They're too big for the puffers to bother, but will they eat the puffers or snap at the tiger barbs? I'm also worried that they'd go after the Oto, since it's stationary much of the time.
I also have a 29 G with two Kribensis, 2 Black Mystery Snails, 3 Peppered Cory Cats, 2 Dwarf Gouramis and 5 Zebra Danios. How would those crabs do in that tank? I don't *need* them since the Corys and snails are a pretty good cleanup crew in that tank, but those little red crabs just look so cool.