Anytime you have a legal sub on the bench, with re-entry rights, he can replace an ejected player. In fact, most rule sets will tell you that he has to take the ejected player's spot.
ive always played local asa so i dont know if its just a field rule, but we say u can only bat 9,10,11 or everyone
you can only have subs if you are batting 9,10, or 11