Good to hear you're getting into goldfish, Blue.
I have a medium sized, "fat and happy" shubunkin goldfish named Ammy who started out as a no name in a group of shubunkins I got from a chain store. After four or five months, he is the only one left, and he is happy as a clam. I have him in a long rather than tall tank, and I think that serves him much better. Nearly every day I drop a slice of blanched yellow squash or zucchini in the tank for him and he devours it. At first I was put off because he wouldn't eat it, but after he figured it out, he loved it. I have read that koi and goldfish in ponds will also eat oranges and lemons, but I do not feed him those because they're acidic and I don't really know if that will funk with the params. They will also eat kale and spinach, but I had trouble getting Ammy to eat spinach after he was used to squash.
While they're primarily vegetarians, they will definitely munch on meaty food, too. I feed my tropicals frozen krill to supplement their flake food, and I usually give Ammy one too, which he will devour.
I've always read that fancies are far less hardy than comets, commons, and shubunkins, so if you really want a fish who is likely to live long, I wouldn't recommend them. True, a lot of times they DO live quite long, but I feel like the other three have a better, more solid constitution. I wouldn't bother getting any bottom feeders in your tank if you have comets, commons, or shubunkins, just because these guys will be better bottom feeders than anything I've ever had (plecos or loaches, for example). I have no idea how much fancy goldfish will do this, but the other three are great at it and will swiftly pick up extra food. I could go even further to say that, while it is more work initially, you could put sand in as a substrate instead of gravel. I had trouble getting my sand completely clean at first, and my tank was cloudy for a week, but once it clears, it looks great, and nothing can sink down inside it. This is great because goldfish poop a lot, so instead of getting trapped under the gravel, it sits on the top of the sand, where it can be netted easily, or if it gets stirred up, it will more easily get eaten by the filter. The same goes for extra food, it will almost certainly get eaten instead of sinking down in the gravel where it can rot and goof with your nitrates. I definitely recommend sand for goldfish, if only because they are good at scavenging and it will be a much cleaner substrate. Goldfish will truly eat almost anything, just keep in mind that they need more carbohydrates than tropical fish, so even if you feed them a good quality food (I recommend Aqueon - slightly more expensive, but better ingredients), it is always good to give them plenty of cooked veggies. Generally, the cooking just helps them sink, but Ammy won't eat squash that is not cooked even if it DOES sink. I wouldn't count on them to pick off any algae from walls or decorations though, just because I've never seen mine or my boyfriend's do it.
I think you would enjoy goldfish that aren't fancies more. I know I've mentioned it already, but fancies are slower and more pokey, while the other three are far more active and fun to watch. My boyfriend's first batch of goldfish (which he still has) were purchased from a tank of feeder goldfish at PetSmart. You would be surprised how many pretty little guys can be found in those tanks, and they're so cheap that if a few die, you won't be losing much money.
For your tank, I would recommend plenty of plants that your goldies won't eat. Anubias Lanceolata was highly recommended to us as a plant that goldfish won't pick at, and from experience, they don't even nip it. Leafier plants that are fern like will most certainly get chomped on, but not necessarily destroyed. I think plants are more important than decor in a goldfish tank if for no other reason that they help stabilize the water params since goldfish produce a lot of waste.
Jo3 is more or less right, I think, on how many goldfish to start with. I think in a fifty five you would be fine to start with four, especially if the tank is allowed to cycle before hand, but it just depends on your preference. So long as they have plenty of open swimming space and they are not crowded by too many decorations or fish, they should be quite happy! I hope this helps, I picked apart my brain to try and remember all the things that owning one of these little goofballs has taught me. Good luck!
P.S. Out of every single goldfish type, I would recommend shubunkins. I've read many times that they are the hardiest, and I personally think they are the most gorgeous!
Edit: Whoops, I told you anubias lanceolata, but it is anubias nana.
