i have had native pistol shrimp in my tank before and they never really bothered anything. They will defend their area (den) but i have never seen them kill or hunt anything. My pistols were the green ones native to the Texas gulf coast. I had a mated pair and one non mated. The pair ran off the single one and they lived on opposite ends of a 55g tank. The thing you really have to get used to is the noise, at first they wil freak you out cause they sound like the tank has cracked!!!!! Trust me, you will get up and look to make sure it hasent at first, then you get used to it.
Good luck hope this helps