Yes, Chris Wallace will be impartial as debate moderator, even though Trump will probably accuse him of showing a bias. He will be an excellent moderator because he knows people view him only as an anchor on Fox. This is his big chance to show he is a better reporter than that.
All of the moderators have been impartial so far, despite their political leanings. While Chris Wallace has a history of bad behavior, he is probably going to try to remain fair and impartial as much as he can during the debate because people are counting on him to be fair.
Chris Wallace will not be an impartial debate moderator. The previous two moderators have not been impartial, therefore there is no reason to believe that Chris Wallace will be impartial. This does not mean that he will not be impartial, but it simply means we must not give him the benefit of the doubt.
Chris Wallace has shown in other interviews that he is biased. He is an older reporter, back from the days when reporters at least pretended not to be biased. However, that doesn't mean that he isn't biased just like any other reporter. Most of the media supports Clinton. Wallace is no different and that will be evident tonight.