It is best if they are introduced together, but they could still pair up. I had a pistol shrimp and a yasha hase goby, they got seperated, but my tank was small and soon they paired up. The ONLY time I saw the pistol was when it was with the goby (although I heard it all the time). I don't think it felt safe to come out when the lights were on unless it had the fish there to let it know when danger was around.