"Actors perform in theaters."
Translation:Aktoroj rolas en teatroj.
No; an aktisto is an official who edits and preserves official acts.
There is not verb "akti" (though perhaps it could mean "to be an act"), and -isto compounds are usually formed from verbs.
Since there is a verb "aktori", you could perhaps use aktoristo if you wanted to emphasise someone who professionally acts.