I just wanted to clarify something-
1. I'm glad your PH meters were wrong- a PH of 6.6 for a long enough period of time can be harmful to the livebearers.
2. Don't add salt. It's good as a medication, or immune system booster or whatever, but a constant dose of salt will not do anything beneficial, other than make your plants grow more slowly.
Also, did you say you had tetras with guppies? Tetras usually (90% of the time) require a low PH, normally batween 6.2-6.8
I also agree that 1ppm of nitrites is 1ppm too much. You're having some kind of problem with you beneficial bacteria- either something happened (the filter was shut off or something) or your tank never fully cycled...
I dunno if anyone else mentioned this, but I would also vacuum the gravel immediately... Could be some dead fry in the gravel polluting the water.
If you're wanting to breed "show quality" or premium or whatever guppies, that requires at least three tanks, and preferably 4.(1 for males, 1 for females, and one to act as the "honeymoon suite". Probably would want a fry growout tank as well.
I breed guppies (if you can call it that) but I focus on healthy fish, not neccesarily blue mostow, albino, cobra whatever- I buy good guppies, and introduce unrelated genetics regularly, but don't worry about maintaining the color or whatever. I cull them by selling the least colorful or smallest tails, but I don't steer the breeding towards blue etc. That way I keep them just in the same tank, heavily planted, with two mail platys as the hunters. Eh, sounds cruel, but the slowest swimmers get eaten, the healthiest specimens survive.
Oh, and don't worry, we all tend to get a bit hysterical when out babies have a mysterious illness. :)