I have put young angel fish in with an older male betta, no problems. Yesterday I put three kribs in the big tank, with a male betta, yo-yos, bumblebee gobies, SAEs, and big grouchy CAEs, no problems at all. However, my rosy barbs killed two new otos last week when I put them in (one barb died, one was extremely uncomfortable for a day. They don't attack otos anymore.) and yesterday they attacked a new cory. I guess a good rule would be, potentially aggressive additions should be the smallest fish in the tank, potential new victims should be the largest. And always buy extra otos.