An indoor pond is a dream of mine, too. Although mine will most likely be a pond in an conservatory. I'll help you as best as I can.
1) Since you are going to be getting breeder-quality TVR then I would allow a minimum 50-60 gallons per fish instead of the normal 10 per fancy. TVR get larger than side-view ranchu and it's not uncommon for them to reach 8 to 10 inches long.
2) Since you don't have to deal with freezing depth (which is what controls how deep outdoor ponds are) you can make it as shallow as you want. I wouldn't go deeper than 3 feet or shallower than one foot. You could vary it in places to give it a more natural look.
3) Barley works in any pond. I used it for years in mine, but it's expensive. And I would always forget to add it in. I just use live plants. Plus,
Goldfish will eat a lot of the algae in the main body. I never have trouble with algae in my pond; it's just on the waterfall and in the stream.
4) For cleaning purposes I'd leave the substrate bare. Outdoor ponds get water changes and cleanings from rain storms. You'll have to clean this pond yourself, so you'll want to make the maintenance as stress-free as possible. Bare-bottom will be better than sand for that.
5) Ranchu are genetically the same as other breeds of
Goldfish. Nothing is more toxic to them than other
Goldfish. I haven't seen one, but I'm sure you can find a guide for
Goldfish pond plants online.
I'm looking forward to seeing you complete this!! It sounds like an amazing project!!
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