2 questons here:

1) what is the ratio for paints to thinner for airbrushing?

2) can i spray enamel paint ontop a layer of acrylic paint?

1) the consistency of skim milk is what you are shooting for. Just add thinner slowly until you get there. For the most part it's about a 1:1 ratio.

2) no problem, I've done it on several occasions, just give the acrylic plenty of time to dry good.

here is another question: is it possible to mix acrylic and enamel paints together?

here is another question: is it possible to mix acrylic and enamel paints together?

What about mixing 2 different brands of acrylic paint? Im thinking of Tamiya and gunze sangyo. Anyone tried that before?

As far as thinning goes.... 1:1 is a good starting point, but do experiment abit as well. Different set-ups and your own preferences dictate sometimes a little less and usually a little more. I dont think a 1:1 mixture is the best solution overall, but definately is around the area where you should start.
I never had any problems spraying enamels over acrylics or spraying acrylics over enamels either. As twisted fate said, just make sure thay are cured first.
Never mix acrylics and enamels together. Acrylics are water based and enamels are oil based. They will not mix.
I cant see why two different acrylics couldnt be mixed together. I have personally never tried it but I mix different enamels together all the time and enamels and oils together as well with no problems