I'm glad I didn't scare you away from goldies. They are some of my favorite fish, but they have such a poor treatment history. It warms my heart when someone does research before buying them. ^-^ They really shine under proper care.
My goldies love moss balls. It is soo cute to see them roll it around the floor. They will also nibble on it from time to time. It's another great plant to have with goldfish. Duckweed is another good plant if you can keep them from eating it all <_< I have little eating monsters. Water sprite is also good, but you need decent lighting for it. I highly recommend plants in with goldfish. I don't know how I managed without them.
Ranchu and lionhead are pretty delicate. When I was first starting out I had one that died about a month after I bought him. He was so adorable; I was really sad. If the only way you could get them would be shipped in the mail, I would hold off on ranchu as shipping is hard on any fish.
Orandas, fantails, and ryukins can easily be kept together. I have a fantail and a ryukin but would like to get another ryukin and oranda (when I get my quarantine equipment back). Some people also keep black (or panda) moors with them, but I wouldn't do it. I also have no experience with seeing-impaired goldfish such as telescopes, celestial eyes, and moors. All I know is don't keep them with faster moving fish especially comets and commons.
It's great that you and your sister want to try out goldfish together. A large goldfish tank makes a great centerpiece in a room (and for a little easier time keeping them happy). That also means you can pool your money and get better stuff. I learned the hard way that it doesn't pay to be cheap with this hobby.
Last edited by thekoimaiden; 01-31-2012 at 08:18 PM.