jksprau, you can use 90% or 70% Isopropyl Alchohol to thin Tamiya Acrylics, but I highly recommend you use Tamiys's X-22A Thinner. I say this because Isopropyl Alchohol dries a bit quicker then Tamiya's thinner and it sometimes will cause a grainy effect to your base coat.
You can not thin Enamel paint with Isopropyl Alchohol or Tamiya's Thinner. If you buy Humbrol Enamels, you can thin it with their thinner or with Mineral Spirits, Model Masters also has their own airbrush thinner, which is also Mineral Spirits.
Again do not use Acrylic thinner or Isoprpyl Alchohol with Enamels.
Rob