I've used other paint and it works but, Testors works the best for me.
Thinning depends on the paint, Acrylic - use water, enamel- use mineral spirits.
I use an eye dropper to drop a few drops of thinner in a jar, stir it up and try it again. Too thick and its hard to work with, to thin and it wont cover well. Best to work on a scrap shaft to work out your formula. I've also noticed one color may be thicker than another and require more thinning.
Getting the proper consistency is one of he tricks to good cressting, makes fine lines so much easier and the paint will level itself out smooth.
Eric