I've had a few over the years and really like them. I've never had one get anywhere close to 11.5" but I've read somewhere that they are capable of that. Mine have topped out between 4-5" like cakeman said. I haven't had any issues with inverts including cleaner shrimp but the cleaner shrimp I've kept them with were larger. They have surprisingly large mouths and they are definitely predators so I wouldn't put it past them to try. Hermits/crabs/snails won't be an issue, just small fish and shrimp.
As far as tempermant is concerned they are normally relaxed and food centric however they can get a little fiesty when defending their territory. It normally just ammounts to fin/gill flaring and posturing. I can't think of any incidents where the harlequin basslet caused any of my other fish harm, really just that the basslet would "take offense" from the strangest fish. For example, my male lyretail anthias and harlequin basslet didn't like one another...sometimes. Occassionally the basslet would go towards the top of the water column to posture at the anthias (who postured right back) and vice versa, the anthias would go down towards the sandbed/rockwork and posture. Like I said though, it really only ammounted to posturing and it was pretty amusing, lol.
Anyway, I think this is an awesome little underappreciated fish. Good luck with it.