Hello Walamma, nice to hear from someone from 'over there'..! Tamiya paints should be thinned with the Tamiya thinner, but failing that, Isopropyl Alcohol will do the trick. As for consistency, everyone will tell you to aim for milk consistency... but you may experience with more or less liquid...
Tamiya Clear is a gloss varnish, so obviously, your models will look glossy. Tamiya has a flat base jar that can be mixed with other Tamiya colours and give them less shine, but I've never been very successful in using that product... I therefore always (other than for airliners and cars) paint with flat paints and cut decals really close to the design (or I paint my markings) in order to minimize 'silvering'.