Whatever you add, you need to add them soon before the CT gets established. After that he won't be as welcoming to new tank mates. I think an eel is the perfect tank mate for an agressive trigger. I have a sargassum and niger triggers with my CT and they get along great. I've also got a sohal tang with mine but that's probably not something I'd recommend since they're both alpha fish. Some people will tell you there is no safe fish with a CT and who knows, they might be right.