24/0 is fine so is 18/6. i personally have never used a humidity dome for seedlings. it just spoils them imo so they get used to the warm moist air then when you remove it they have to adjust to the new environment. i just germ the seeds, put them in a 120z cup of promix/perlite and watch them grow. i never used a heating pad for seedligs either, just for cloning. good luck