A single fish in a 29g tank is kind of boring to look at, but I have a pair of 20g tanks each with 3 GF. I have good filtration, I do not overfeed, I use a bubble wand (which increases surface agitation) and mine have done quite well. I've heard the 30g rule for GF, and I find that a little extreme. 10 years ago in the pages of Aquarium Fish Magazine, Steven Meyer, who does their ponds and GF columns, was recommending 15 - 20 gallons per GF, and since then, the number has been rising steadily. I wouldn't be surprised if that number reached 40g soon. And this as filtration has gotten better and cheaper over the past decade. I'd think you could add a pair of any other fantail variety that has a dorsal (top) fin: moor, oranda, redcap. This would make an interesting counterpoint to what you have now.
If you have any specific questions about food/filtration, drop me a line...
Val