Osan, huh? I'm AF myself, going to Kunsan in a couple of months.
For thinning, each brand generally has their own thinner that is used with their paints. You don't necessarily have to use it, though. For acrylics, I usually use plain ol' distilled water (or if I'm feeling extra lazy. . . tap water.) You can also use denatured alcohol. The 'rule of thumb' I use for thinning is get it to a milky consistency. Depending on which Testors brands they have available, you may have to thin a little more or less. Personally, I've found that the Model Master Acryl line doesn't take as much thinner as the 'standard' Testors paints.
If you're mixing custom colors, then you should always mix acrylics with acrylics, or enamels with enamels.
For painting one on top of the other, I've never had any issues with painting acrylics over enamels. Just make sure your base coat is nice and dry first. For enamels over acrylics, I've done that too. I just put a clear coat in between to avoid any potential issues.
Eric
I find your lack of faith disturbing. . .