Tbh, you can't be sure he's mellow enough unless you tried it before-I had my cories with my bettas for a while and decided to put them with my most mellow (or so I thought) male, Kyle. He was flaring like crazy and almost hurt them. I moved them with my other male, Kenni, he was supposed to be the agressive one but instead, he left them alone. You can try, although I wouldnt recommend it, but make sure you have a tank handy in case something goes wrong.