I dont generally encourage peacocks as a reef safe mantis just because they get too big, so make sure you are prepared to upgrade when it does get too big.
well... generally at least. I really dont encourage it at any rate. Right now Im sort of experimenting with that with my 5 inch peacock in a mixed LPS, SPS reef. Softies seem to be of almost zero concern to the mantis. It hasn't been long in the reef, maybe a month or so, but it hasn't caused quite as much damage as I thought it might. The only notable damage was taking a chunk out of a green monti cap that was a bit too close to its burrow. Of course im still going to let this tank run for a while longer to make sure; long enough at least to see how it fares with larger coral colonies. For now, Im still sticking to the "peacocks are not reef safe" point of view until I have a decent experience to say otherwise.