bro, transmission fluid in ist gen vitz is good for 100,000 km under normal driving conditions. so are you sure you want to change it?
about engine oil, toyota recommends 5w30, 20w50 should be too thik imo.
about spark plugs, change them only if you need to, and i have installed iridium plugs, they do last longer than regular plugs, but i did not notice any difference in performance (neither power nor fuel economy) so i think there's no need to go for fancy plugs in this kind of car, just get some good quality regular plugs