The Golden Guppy's benefit as a Magic Fish is the fact that it sells for several times as much money as any other fish. Unlike the other 6 Magic Fish, it does not affect the other fish in the tank.

The Twin-Fin is one of a handful of fins and bodies that cannot be produced by breeding. The only way to get more Twin-Fins (apart from buying and hatching eggs) is to breed two Twin-Fin fishies.

I have observed that the tendency is to breed the same fish over and over, which can result in missing a lot of species! The only way I know of to avoid that is to adopt some sort of systematic breeding approach. If you don't want to resort to tracking your breeding on a spreadsheet or...gasp....on paper, you can use a simple approach that usually produces fishies that you've never seen before:

Make sure that you have level 3 of Fish Environment before you do this, since it will produce fish that can't survive in level 1 or level 2 Fish Environment. Start with a tank that has 20 fishies of various sorts, and have another empty tank. Pick one fish to be your "breeder" fishie and breed it with every other fish in the tank. Make sure to divide the pregnant fish evenly between the two tanks, so that you don't "lose" the babies. Once the offspring are born, sell all of the adults except your "breeder" and repeat the process with the offspring. Continue doing that with each successive generation until you start getting repeats, and then choose a different breeder for the next iteration of the process. You'll be surprised at how many new fishies you can discover that way. No tracking required!