Jeeves,
I used to do some side research in paint pigments years ago (not hobby paints, but they were acrylics). As I recall, various additives are available to "stabilize" paint. In unstabilized paint the pigment particles will naturally begin to adhere to each other over time and settle out into a hard-packed bottom. Polymers and additives are used to keep the pigment stable so it will remix into a fine, even suspension.
The stabilizing agents used by one manufacturer will not necessarily be compatable with those from another manufacturer. When you mix paint from different sources, you may get an unstabilized suspension, leading to grains, higher viscosity, etc. If this is the case, I doubt any thinner will help much.
I try to mix colors from the same manufacturer, or at least test a mixture before airbrushing. I put a couple of drops of each color on a sheet of plastic wrap over white paper, mix them together with an old brush, and spread the paint thin on the wrap looking for grains, clumps, or incompatable mixing.
I have observed this phenomena only in acrylics, which are suspended in a polar medium like water. I don't think I have ever had a problem mixing enamels, though I'm not saying it can't happen.