Betta Fish bothering cory catfish

My husband and I very recently started a 15 gallon tank. We first put 4 Cory catfish in it and then we added a beta fish the same day. This was Saturday 1/14/23 and we noticed the Betta will not leave the corys alone, chasing them and nipping at them. He seems to leave them along a Little after he’s been fed. We have an air stone in the tank as suggested by a few people at our local pet stores and we cycled our tank first. The betta keeps hanging out in the bottom half of the tank so I think the current is too strong. I just don’t know what to do. If we have to we’ll purchase another tank for the betta but we don’t know how long to give the fish to adjust. We don’t want to stress him out by changing tanks again but all this flaring must be stressing him out too. How long should we give them before we decide to separate them? If we do need to separate them how would we do that in a way that won’t potentially kill our Betta?