I would think these 2 would get along, they are both technically considered puffers. Maybe if the porcupine is a bit bigger, he might pick on him, but I wouldn't think they would for long. I have a cowfish, but I have never had a porcupine in with it, but my dad has had both and they got along fine. But the cowfish he has was full grown, but his porcupine was also pretty big, so who knows. Just keep an eye on them the next few days, and if they keep fighting you might just have to take one out. Also like Bama Reefer said, I think it would take quite a bit to stress the cowfish out because the first night I got mine, he got stuck in the pump intake and I pulled him away from it and hes been fine ever since. The poor thing may have been stuck there all night for all I know! But he didn't get stressed at all, when I unstuck him, he swam around right away looking for food as usual! lol They are fun fish to keep.