Thanks Keith, been in the hobby of fish keeping for many years and it's nice to be able to share the knowledge with others that can possibly use some of it for their benefit.
Unfortunately I have never dealt with raingarden. I've only seen a few dealers carry, Wakins goldfish, I'm assuming that watonai and wakins are similar as this is actually the first time I have heard of Watonia. Basically a Comet with a fan tail? I actually went and googled the fish while typing and it seems to be a wakin with a larger tail. My local dealer has about a dozen wakin in a small pond. Cool fish, I personally like long fin straight tails. We have a few fancy Ryukins in the bedroom aquarium. I tried a few in my pond but they didn't do well. The large Comets constantly chased them and stressed them severely, they kept getting caught in the skimmer, and I found out Orandas, and cold water below 35 degrees will kill them. With all that said I wish I could help more.
By the little reading I did , I would say they are probably best kept with similar fantail type goldfish or smaller comets and shubunkins if at all. Minimal water circulation, or at least not a real strong water flow. Also I would recommend plenty of lilies or shade for them during the summer and a place to keep them fairly warm,(50-70) during the winter. One thing to keep in mind and be aware of with any fancy fin, fantail, or long finned fish. They are more susceptible to fin fungus/ fin rot. the reason is the blood vessels are very much reduced going to the extreme ends of the fins. GOOD water quality is a must, Not a bad idea to have a good anti bacterial/ fungus medication on hand in case it is needed as you will need to treat quickly if a problem does occur, and have someplace to treat. thankfully fantails of any type don't get extremely large so setting up a hospital tank for them can be as simple as a aquarium, clean garbage can, kiddy pool, etc. unlike Koi which need a much bigger area to treat them. Good luck! I'd love to see pics of them when/if you do!!