Goldies, particularly comets
@Mmathis
The only thing I know is that GF that will grow up to be “normal color,” usually start out as an olive color with non-descript eyes — very drab. They can be very dark, almost black before they “turn,” but not always. If they will be Shubunkin they’ll start out pink with black eyes.
As far as fin/tail length, I can’t help there. All of the babies I’ve ever raised turned out to be commons, comets, or Shubies — or hybrids, but I don’t recall anything in particular about fin/tail length — when that became evident, etc. I have 3 very dark GF right now that have been almost black for about 2-3 years — all spawned in my pond. Two of those have a bronzy-cast, but “normal” looking tails, and one is more true-Black with a longer tail. I keep expecting them to change color any day, but I’m pretty sure the fin size is whatever it’s going to be by now. They are probably in that 3” range.
When I raised my fry a few years back, I did a LOT of research on colors, etc., trying to anticipate what my babies would turn out to be. There’s not a whole lot of info out there, but you might try a Google search and see what results you get. It is interesting, but I can’t tell you if what the fish guy told you is correct or not.