Actually Pistols can become very aggressive in a pairing attempt and it is not always successful, sometimes it ends in a death. Some pistols are easier to pair than others, some wont pair up. I know ive heard many people being more successful with tiger pistols and years back that is what i had paired with a shrimp goby. I bought them paired and never had a problem. Both came from a well established tank though.
I would bet 100% it being the pistol both times.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.