When stocking goldfish, you should always think that 20 gallons is the minimum for one goldfish, but you can also keep two goldfish there and so have 10 gallons per fish.
If you are prepared to be taking good care of the water quality and maybe upgrading the filters in the 29, you can fit 3 goldfish in it
If I where you, I'd of course keep 2 orandas, since they grow really big, and they'd love the extra space in the 29
Or you could get another fantail friend for your current one and let the comet move into the bigger one with a buddy
That would fit his need for room to swim a bit better