No. The cop was way out of line here.
What in the hell justifies him going 100 mph? He had no evidence of a crime. No one was being assaulted, murdered, or kidnapped. That right there makes him unsafe to be on the streets. He had NO business going that fast.
Second, if the red lights were authorized by his organization, that makes him an emergency vehicle in Wisconsin. The law actually says that if he's responding to an emergency call, the use is appropriate. If the police department has an issue with how the fire department is interpreting the law, that's an issue for administration of the two agencies to deal with; not on the side of the road.
http://statutes.laws.com/wisconsin/347/347.25
Third, this guy had every right to believe they were going to the same emergency. And as the law states, he was an emergency vehicle, responding to an emergency.
Fourth, this hero with a badge knew when he pulled in to THE FIREHOUSE that the guy had legitimate volunteer plates, and that there were other members firing up the equipment. Pulling his gun was purely out of anger that someone disobeyed him. Common sense should say that this is a volunteer going to a call. A cop better have a really, really good reason to pull their gun and aim it at someone. They're supposed to be more level headed than that. This guy is a loose cannon. What if he had shot him? How in the world could he justify that?
The cop escalated this situation needlessly. He endangered lives without good reason. 100 mph chase and pointing his gun at someone's head because he's angry at him? Bet your rear end I'd be suing too, and I'd do everything in my power to see this guy stripped of his badge.