This is a challenging breed to work with, Joann. There simply is not much good stock out there, period. The tail barring in your pullet could actually mean better tail barring in her sons, but obviously, she should have tail black, not barring. That pops up even in good breeder lines. My Phoebe had some barring in her tail as a youngster, but now, has very good tail black instead. You really don't fully know what you have with these since it takes until maturity for them to get their final plumage and clear out the excess black color on the backs of the males.
In Delawares, it's best to do a double mating system to get the best birds, however, that obviously becomes a bit more complicated, takes more space, etc. Of course, if you have a perfect Delaware already, you have arrived. It is quite a process to breed them for both sexes to be correct per standard.
IMO, the best thing to do is to start with stock from a really serious breeder of Delawares (if you can find one, that is!) and go from there.
Cher, I'm glad you ended up with a great big guy out of that bunch! If folks saw Isaac in person, they'd be amazed at how really huge he is. Even standing near my great big 14 lb. blue Orp, Suede, he doesn't look small in the least!