Can you describe more on their appearance? Do they have any head formations (like bumpy forehead) or very large eyes etc? They are probably a fantail/fancy goldfish of some sort like a Ryukin or Pearl scale .
This link may be of some help ID'ing the goldies;
As far as fancy goldfish go, you should have at the very minimum 20gallons for the first goldfish and 10gals for every one after that- i know it sounds a lot, but goldfish can grow suprisingly large (many types can grow over 8inchs long) and they are also big waste producers, so powerful decent filtration is required.
Non-fancy goldfish like comets and common goldfish are really not suited to tanks at all due to their activity levels and size (comets can grow to over 14inches long for example), so most people advise them only for ponds unless your tank is going to have a lot of gallons/length. Non-fancy goldfish and fancy goldfish should not be mixed, because many of the fancy goldfish types out their like Pearl Scales are too slow to compete for food with the sleeker types of goldfish and are often prone to being bullied or nipped because they cannot defend themselves easily.
Goldfish are sociable fish and really do prefer to be in at least couples, so if you plan to get goldfish i would get at least a couple (if you can house them properly) as it will improve their quality of life a lot just as much as having enough tank space to house them will .