I wouldn't recommend using feeder goldfish to cycle your tank. Feeders are shipped and kept in overcrowded conditions and usually are carriers of all sorts of nasty diseases. It is a sure way to give your tank Ich.
Besides, goldfish like cool temperatures, and the fish you want to keep are tropical, warmth loving fish. Once you switch over to heat, it will throw off the helpful bacteria colonies you have been trying to establish, resulting in another ammonia spike and more time being needed for things to balance out again.
You would be best off buying a small number of fish at a time. For a 55 gallon, I'd get about 6 at a time, wait a couple of weeks, then add another 6, wait a couple of weeks, etc. Platys and Swordtails are hardy fish and would be a good choice to start cycling your tank. Wait on any tetras until the end, as they are more delicate in regards to water quality. If you want fancy guppies
and danios, make sure you have a school of at least 6 to 8 danios. Unless they are kept occupied in a school of their own species, danios can be quite nippy to other species and with the long tails, guppies would be a prime target.
When figuring out how many fish your tank can hold, keep in mind the adult size of the fish you get. For example, a fish bought as a juvenile at 1 inch may grow to 3 inches. You will need to account for that when planning.
Ideally, it would be a good idea to have a quarantine tank you can use (usually a smaller, 10-20 gallon tank). This way you can observe each batch of new fish, to make sure they are healthy, before introducing them to your larger tank. It may seem like an added expense, but when you consider the cost of medicines to treat 55 gallons of water vs. 10 gallons, not to mention the cost in losing favorite fish and replacing them, a quarantine tank pays for itself many times over! I've done it this way for over 20 years with very good luck. I even have a catfish that is 18 years old. Anyfish added to her tank are closely observed for at least two weeks before introductions are made!
Best of luck with your tank!