Hunh. Well, here's my take on things: If you're getting hermits for CUC, they don't belong there. Snails do better. You can get Nassas that'll stir up your sandbed, astreas and turbos that'll clean off algae, etc (ceriths are useless, IMO). In fact, unless you have a large predatory fish that drops chunks of flesh around, you don't really need a hermit crab.
BUT: I find them interesting and adorable. I've kept the "worst" species with no damage to corals or snails. Shrimp are more terrible than hermits at feeding time, IMO - the peppermints will stop at nothing to pull a piece of food out of a duncan head. But you do need to feed them, unlike snails, because if you expect them to just live off detritus and algae than you'd better be expecting aggression. Also, I take the whole "hermits killing everything" spiel with a grain of salt - I've found that mine are able to sense a freshly deceased fish (within the last few minutes) and absolutely demolish the body in about ten minutes.
As for species - Electric Blues are one of the gentlest hermits. They do have a habit of holing up in a crevice and staying there for ages - the first few times this happened, we removed the hermit from the crack and he crawled right back into it :/ They love grazing on chaetomorph, and would rather tuck into their shells than fight. They're also very slow growers, and won't get to two inches any time soon. Mine seems to have capped at about an inch, maybe an inch and a quarter.
"Dwarf" zebras will get about two inches. Left handed hermits get larger - the largest in my tank is at least two and a half inches. Both are food aggressive and will fight over scraps, but otherwise I've never seen them bother anything or pick at corals. If you're feeding something else, drop a good-sized pellet in for them to fight over first, and you shouldn't have any problems.
