They will breed with each other. They will form the semi-fancy guppies (which I like to call them). There will be mixing of colours from the wild and the fancy guppies. For example, the ones that I have bred were mostly blue, but there were a bit of orange on the fins. Most of the stock died when the Female Albino Kribensis decided that she should have the whole tank as her territory...
Not recommended if you are planning on selling them, as it would be hard in tracking down which fish is which. Semi's seem to have almost the same colouration and would be difficult to distinguish from the fancies.
I would suggest getting only fancies or wild, but not both.
***OT: only have 2 F2 remain and they seem to be semi-fancies (after the breeding of semi-fancy x semi-fancy).***
According to another forum member, a fancy and a common breeding would result in a 100% semi-fancy offspring. Once you breed the semi-fancy offspring with each other (so F1 x F1), you will only get 25% fancies, 25% common and then 50% semi-fancies... It would be easy to differentiate the commons from the fancies and the semi-fancies, but it would be difficult to differentiate the fancies from the semi-fancies.
So, I would say that it will be better to breed fancies with fancies rather than breeding a fancy with a common since it would I guess take less time to get 100% fancy guppy babies as it would only take one live-birth from the female rather than three generations of breeding guppies (semi-fancy, fancy, common, hybrids).