I have my betta with a school of neon tetra. He super mellow with the fish but is a snail assassin. Each betta has its own personality. My recommendation is for you to get the other fish first and give them a week to acclimate and "own" the tank. It seems to help to do it that way if you have a mellow betta because the fish are not new to him, the whole tank is. Plus the fish know where to hide. If you add them after he already has had a chance to establish his territory, the new fish are more likely to end up as targets. Adding the betta last worked for me. However, when you get your betta, be prepared to have a separate tank for him on the side in case he's a 007. You really never can tell.