I don't hesitate to use "kočka" for a tomcat but I'm quite reluctant in the case of "kočička". I use "kočička" for female cat only (male cat is "kocour", diminutive "kocourek"). "Kočka" can be also "beauty" or "stunner" ...and "kočička", to extend Glenn's post about Czech-English similarities, can be "babe", "pussycat" and also "catkin".
