Although Nazism developed alongside fascism, they aren’t quite the same thing. Mussolini and the fascist E. European governments cooperated with the Holocaust but they didn’t instigate it on their own. So, even if a fascist government were to come to power in the US (and Trump is not a fascist) it doesn’t follow that there would be a kristallnacht.
The “rally” you’re referring to was actually a conference of Richard Spencer’s National Policy Institute. The NPI event drew about 200 people.
The real fascist threat these days comes from the SJW left, which has mobilized mobs of people to riot, block people with contrary opinions from speaking, hounding people who disagree with them out of their jobs, and forcing the closure of businesses whose owners disagree.
Many believe that the election of Trump was a narrow escape from left wing fascism. The fear was that the extremist left, which has taken over and subverted academia, would gain power over all other American institutions.