cross. The plumetail, coming from regular parents likely had one gene for regular fins and one for plumetail. Plumetail is dominant over the regular gene, something a bit unusual in fish sports.
So only some of the off spring might be plumetail. It will be interesting to see. Probably available somewhere on the net is whether regular fins are dominant over the gene for highfin. My guess is that they are and that you will have to wait until the second generation (back crossing the youngsters) to see hi-fins. If you were using a 100% plumetail, that might still be the case.
Having been out of town five days and still sorting things out, I've not been keeping up here. (Delighted to see the maintenance here though!) How about if both of us look around for platy genetics and hi-fin x regular fin platys?