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Any goldfish with a double tail is considered a fancy goldfish although theres exceptions to this rule and breeds that cross lines between double & single tails. Common goldfish are what you typically see in feeder tanks at the lfs.
In a 55g? Right now, you could manage another 2-3 fancies. Larger adults would be a bit cramped in a 55g at this number though. Make sure you have hefty filtration (min 550gph) and stay on top of your water changes. Its always a good idea to understock with messy goldies.
If the filter is 'meant' for a 55g, then its likely not enough. You want filtration in the 550gph+ range. You may need another filter to reach a higher capacity. I run 2 hob's 'meant' for 90+g tanks on my 55's.