I have my 3.5" GT in with Rosy Barbs and Columbian Red Fins at the moment in my 3ft. However she will be going in my 5ft with my O when I get it. I think long term barbs would be risky, my GT doesn't particularly like the barbs but tolerates them at the moment.
Are you going to be getting a larger tank for the GT eventually?
When you say "O" are you talking about an Oscar. If so and if you terror is anything like mine its not going to work. tried putting my smaller green terror in with my oscar and 5 clown loachs in my 6 foot tank. Well by the end of the night had to take the terror back out. All that space and the terror thought he would back my adult oscar who is twice as big as him up in a corner and try to make him stay there.
Yes I mean my oscar. My O is about an inch at the moment and my GT is about 3.5". I've had my GT since she was less than an inch, so the terrotory is well and truly hers. However I put my O in and there has been no distruption at all - not even at feed times when normally she chases every other fish - she never chases him and he doesn't chase her.
The thing with cichlids is you do not know how they will behave as they are all unique. I have read a lot of people have kept o's and GT's together harmoniously, and my two are fine at the moment, even in a tank which is too small for them. I am getting a 5ft shortly.