I think that the convicts would work, but by adding another cichlid regardless of size, you are adding aggression. So maybe 3 isn't the best option? Not sure! Like you said, you have extra tanks if they need to be relocated
I think what a lot of people miss, is the beauty of keeping just one large cichlid in the tank, with some other non cichlid tank mates (acceptable ones of course). Its frustrating waiting for that little pale, new, and slow fish to get big and colorful, so people add more cichlids to get some interaction or they want more beauty. In the future, you'll wind up with the fish always fighting, torn up fins, hiding in the corner. I'm certainly not saying that os what you are doing, but that's what I see a lot.
Sometimes the it is better to have a 1 cichlid tank. You will definitely get a lot more interaction that way. Let's say you got the GT and it is just sitting their all shy and pail. Just wait a week, it'll come out more, but still probably be kinda shy. Wait like that for a month, until the fish is a little less scared of you. Then get a school of silver dollars. The GT will feel like it has competition for food, and will start definitely come out more. Plus the look of a school of silver disk looking fish, sweimming next to a green and blue fish, with gold fins, just looks awesome. Then if you feel the need to, get a pleco

maybe bristlenose, but from what I've experienced, they are a little more fragile. I may just go with a trio of pictus cats for the bottom

that would look cool!
Just keep that in consideration! If you go with the convicts, you will see more of the fish's aggressive behaviours, but they will be so worried that it will get attacked that it wouldn't interact with you as much :/
Good luck!
Just on a completely random note, I just got some fish today! And I am seeing how eels work with my oscar and JD. My tank has 1 oscar, 1 jd, 1 peacock eel, 1 Geophagus, and a smaller pleco in it. The geophagus was instantly attacked by the jd. And the Oscar actually tried eating it! It held its head in its mouth

so I have to take him back soon :/ and the jd doesn't like the eel that much, but the oscar does not care
