My 9" Oscar with two female opaline gouramis, one 3" one 2"
When I first introduced the oscar to the tank at ~3", all the other inhabitants were picking on him and forced him into the corner, but then the larger of the two gouramis came next to him and they made eye contact, the gourami made his colors dark and protected him until the oscar grew some balls and kicked the **** out and or ate the rest of the fish in the tank (including a 5 inch common goldy, 3 3-4" gold barbs (one of which he bit in half and left to rot), and a 6" gt)
And today, he lets them do whatever they want in his tank, they even "clean" him sometimes, which i find really weird. He just stays in one place while they pick at who knows what on his skin and anus.
Whenever they get in the way of his food he just looks at them and waits for them to move so he can eat. They're bite size for him, but he loves them.