Olympia and LBF have hit on some of the main points, but I will reiterate a bit. I consider 30 gal as the minimum for goldfish. Really any smaller and you're going to run into physical issues as well as nitrate control. Of the goldfish that could go in that tank, you're going to want to look at the more physically modified fish like celestial
, and bubble eyes
. Those tend to stay smaller. They also have more trouble swimming so they don't need as much room.
As Olympia and LBF mentioned, the single-tail goldfish won't do well in anything under 100 gallons. Really they won't do great in anything shy of a large pond. That being said, there are calico varieties of other goldfish like ranchu and ryukin.
As far as another fish in the 30 gal with two goldfish, I think that's pushing the bioload too much. You are already going to need to have a large filter to take care of the goldfish waste. A lot of bottom feeders are high-waste fish. Plus, pleco have been known to suck on goldfish's slime coats.
If you have soft water, you might have trouble with goldfish. They are a hard water fish. In soft water, they can experience slime coat loss. Without a high KH, the pH of the water can experience rapid drops as the week goes by.
By telescope goldfish, do you mean these? Those like oranda and fantails tend to get pretty large.