I had about a dozen small aiptasia on a large rock that I bought from another reefer. I added two peppermint shrimp and the aiptasia were gone within a few days. I called around to local LFS and asked them if they for sure had peppermint shrimp. I trust them and so far so good. One of the shrimp is much bigger than the other and eats like crazy. This is also the one that has bothered some of my coral. I'm guessing that is also the one that took care of the aiptasia.
Here are some of my observations:
1. On two different occasions when adding euphyllia, this shrimp went crazy. As soon as the coral where in the tank, the shrimp was flying all over the tank and came up to the coral and appeared to try to eat it. I don't know if it wanted to eat the coral itself, the slime coat, or something that was on the coral. Both times I aggressively chased the shrimp off and back to his cave. To the best of my knowledge, it hasn't bothered these coral again.
2. The large shrimp will go after food anywhere - on the surface of the water, in coral, etc. It will pull mysis shrimp out of coral if I let it. When I feed my tank, I put mysis shrimp in the back of the tank for the fish first and the shrimp go after it there. I then feed the coral. I have to guard the coral the whole time or the shrimp may eventually come up to steel mysis out of the coral, especially from the duncan. I always aggressively chase the shrimp away with tweezers. The smaller shrimp does not come up to the corals anymore after I chased it away a few different times.
3. After these shrimp expire, I won't get more unless I have another aiptasia issue. Shrimp appear to be a bit of a gamble and a nuisance in a reef tank (this is the only shrimp that I've kept, so I'm making some assumptions here about other shrimp). Anything capable of eating glass anemones is probably also capable of eating other things.
Sorry for the long post...