Aww thanks! I'm enjoying writing it. ^-^
In theory a ~30 gal tank is large enough for two goldfish, but that would be maxing out the biological capacity as well as the space. Some breeds like oranda and black moors would get too large to be placed in a tank of that size with another goldfish (I've seen some monster oranda that take up a whole 30 gal by themselves). Unfortunately there is no way to tell how large a goldfish will grow; some breeds have general expectations, but individual fish like to break those ;P. A single goldfish in that tank would be better, but you would need pretty heavy filtration (still try to find a breed that will stay small-ish like ryukin or ranchu). I would skip the HOB and go right for a canister. Live plants will also help. You're also going to need to do a lot of water changes. So to sum everything up, yes, a goldfish could work in that tank, but it would have more maintenance needs than a regular tank.