I love ryukin goldfish, they are so beautiful, especially the red and white ones! I think 2 could work in a 29 gallon by themselves, if you have a good filter. I don't know if they would do well with a betta, however, as the two have different temperature requirements. Perhaps you could get a separate tank for the king. Whatever you choose, I hope everything works out!
Ares - My flare happy warrior
Castiel - My inquisitive crowntail
Sammie,Jackie, and Jamie - My fun loving peppered cories.
Touchstone - My sweet fool
Freyja - My beautiful girl