1. From the information you provided in your first post, its sounds as if your model needs priming before trying to put on a topcoat, especially if your are experiencing holes in the paint. Paint surface tension is causing it to bead up on the surface, much like car wax. Get a good flat and shoot it on the kit, then try for the topcoat.
2. Robert is right. A lot of the Tamiya colors can be sprayed straight out of the bottle without thinning.
3. Gloss coats may have to be applied in thin misted coats, particularly if the colors are light (say for example, gloss white). You might find it easier to spray a flat coat, and then use an acrylic overcoat, like Future, to create a gloss finish.
4. As far as how thin the paint should be, you are going to have to experiment to find what works for you...but then, that's the fun of the hobby. Hope this helps.
Gip Winecoff