In Luke 9, Jesus says whoever is not against us is for us. Can that apply to Protestants?

Yes, in a way. Paul says something similar in Philippians, where he has competition from people preaching a gospel that is not exactly what he preaches, but at least the name of Jesus Christ is mentioned, and there is a chance those people will follow it up and become saved; God can use that too, because the name of Jesus is spread. So Protestants, Jews, even Muslims who think Jesus was a good prophet, if somebody hears the name of Jesus Christ, that means something, and it is enough to get them started on the road to salvation. Of course people can also be deceived and not be saved after they hear the name, but that does not stop us from preaching it, because as 1 Timothy 2:4 says, God wants all men to be saved. God has already put the law in their hearts, according to Romans 2, so that nobody has an excuse before His judgment seat. Paul's attitude is, I am going to accelerate the process by preaching the name of Jesus and building the church wherever I go, to get as many as possible into the fast lane. So yes, any preaching of the name of Jesus is a good stimulator.