That's the biggest challange you'll find, even if you understand the genetics of the situation. Finding pure strain guppies is next to impossible at the LFS. They are usually special order items, expensive, and the only way to tell if you got pure strain is to breed them in an established cross, then check your results. Add to that, female guppies store sperm for months, and you wind up with the same brood time after time, even after the male has been removed.Guppies were aptly named: Million-Fish
The genetics is very difficult, and unless you spend the $ upfront for clean lines, your efforts can get very frustrating...
I did enjoy hybridizing them. Ran my own genetics experiment for about five years, but I'm outbreeding now, with fresh stock. Too many culls had to be thrown to the predators.
In five years of messing around, I wound up with females, larger than average, with a golden hue, instead of a silver one, and guppies with an big top fin display, and a tattered back fin appearance. Kinda like comb, but not quite. Also got their temp comfort zone down to about 60 degrees. I'm still getting endler variations, but my basic stock mostly stabilized.
