I do miss one thing that you haven't mentioned, that's the genetic background of the first breeding pair and the coming generations. It's better to know the genetic history of both breeder fish (so their genotype) than using their phenotype as a foundation to set up a new line. For a certain phenotype doesn't mean that it's the same genotype as a logical source.
You can use the mendelian system but that's just a theoretical system that can be used but won't always be accurate if you want to linebreed. But it will give a clear diagram of how it should proceed. Because this structure of breeding is not a 100% accurate, selecting is in order in every next generation. Which reflects a bit of what you've been saying. If you have crossed two types of guppies, assuming that the female used is a virgin, the offspring will have genes of both parents. But the combination of those inherited genes will make the phenotype of the each individual offspring. Which mean that the whole offspring will have the same gene combination or different. And which genes will be dominant, double dominant, triple dominant, recessive, double recessive or triple recessive in such a gene combination. This goes for both male and female. And is a certain trait, X-, Y-linked or autosomal? But as already been said by me, a certain phenotype doesn't have to mean that it's equal to the genotype.
Buying a female guppy from the store won't guarantee that she's not storing any sperm packets from a former mating or matings. They can stores perm packets for over a year. As you already mentioned, use always virgin females.
The way you've been saying it, needs more generations to get the job done. And that they will breed true. It's better to use a female guppy where you know of that she will pass the traits of her own strain recessively. That will put the number of generations down to create a new strain that will breed true.
And if you want the desired trait that those males have in your future females, use females of that line that aren't XX females but YY females. Such females will show the same traits (less present than in a male) as those males. And those traits can be seen in the fins and partially the saddle or caudal penducle. That's also the last step of linebreeding to own a true breeding strain.
I'm a huge guppy fan as well and I do use linebreeding as well.