The pistol would probably only be at risk right when it is introduced. After introduction, it will most likely find a place and dig a home pretty quickly which should keep it safe. I don't think other inverts should bother it and in fact may need to worry more about the pistol harming them. Biggest threat would probably be fish, such as certain wrasses, that prey on inverts. You could minimize this threat by introducing the pistol at night after the fish have gone to "sleep".
Two things you may want to think about before purchasing if you haven't already:
1-Pistols like to dig which may cause a problem if your rockwork is not secure and/or is resting on top of the sand rather than on the tank bottom.
2-Not really a problem per se, but in a larger tank such as a 125, there is real possibility you will never see the pistol after you put it in. They spend a lot of time underground or very close to their burrow entrance which may be something to consider before spending the money (JMO).