I'd set a 20 long up using RO water, air driven sponge filter, and minimal if any substrate. Any driftwood with mosses on it you already have would be a great addition. Get it going for a month, don't sweat any film algae. Start feeding the adult fish live BBS, frozen cyclopeeze, any tiny live or frozen foods you can get your hands on. Hopefully you'll start seeing some rounded bellies on females pretty quickly. Drip acclimate these females and your long finned male to the new tank if your tank water isn't as soft (I doubt it would be). Dim lighting and check the fish daily. Return any females who seem to have laid back to the main tank, and the long finned male with the last one to scatter her eggs. Care for fry as with any small tetra or characin.
What I don't know is whether the gene in these is simple recessive or line bred. If simple recessive (which I suspect), either half or none of the offspring will have long fins, depending on whether or not any of the females was heterozygous for this trait. But don't worry, 100% of the offspring will be. However, you will have to grow these up and breed them back to that male again to get substantial numbers of long finned offspring. If you get any long finned fry, any females should be bred back to the father. If these are line bred, I would expect to see some growing fry with longer finnage. Select these females to breed back to the father. It isn't going to be instant, but it's worth it for a first.
Nothing good happens fast in an ecosystem.