The simple rule is: don't mix enamel, lacquers, and acrylics. (While in liquid state.}
Use enamel thinner to thin enamels, laquer thinner to thin lacquers, and for acrylics, use the thinner made by the manufacturer of the acrylic you are using. For instance, use Tamiya acrylic thinner for Tamiya acrylics. Other brands of acrylics use different formulations of thinners which may or may not work if you cross mix them.
The cheap craft acrylics you can buy at places like Wal-Mart can be thinned with plain old water, but I would not recommend their use, except for plaster figures, wood plaques, and stuff like that.