That's a lot of fish for a 55G. My two, which are probably half that size are in 75G. Minimum
for a goldfish should be 20-25G per fish, and more is in this case better.
Also, I'd skip the flakes. I find flake good only for small, surface feeding tropicals. Much of it falls down, isn't found and just pollutes the water. As for feeding, I've got one fantail type that's fast and smart. The other is a bumbling, ungraceful Oranda. When I feed, I tap the lid on the glass at the same spot and the smart one comes up and lets me feed him. I make sure he's got a good mouthful, then I drop the shrimp pellets in the area of "Miss Graceful" so I don't have to toss in extra to cover bases as it were.
They will always try to convince you it's time to eat. Do not fall for it. They can go many days without. In fact, enforce a once a week no feeding day. It's good for them.
One goldfish (telescope) is quite large (and round) - about 6 inches long. As big as any I see in the stores (if not bigger) - I also have a large ryukin about 4-5 inches and two others about the medium size - 3-3.5 inches. They are in a 55 gallon tank. The water is clear. The algae is on the glass. very green. I use the magnets and a scrubber alot - daily. I change the water weekly about 50%. The nitrate level is low and the ammonia and nitrate close to 0 at all times. No chemicals used at all because I use private deep well water. PH is on the high side naturally - mid 7.5 or so.
All artificial plants, feed them oranges, shrimp pellets, flakes, sinking foods, peas. I probaby do overfeed them, but they always seem hungry, except the telescope who has clearly challenged vision.