10g, betta, 5 male guppies + 2 endlers, fin biting on several including betta?

A couple of days ago I noticed fin biting on one of the guppies, now I wake up and 3 of them have fin biting as well as my betta. 1 week ago I added the last 2 guppies to the tank including the new betta, he seems pretty peaceful when around them, but there hasn't been any issues up until 2 days ago. Perhaps the new aggressive guppies mixed up the order.

Any suggestions on locating the problem and where to start?