"When is it okay to compare somebody today with Nazis?"
When the comparison is actually valid.
The word “fascist” has been used many places for many years, often as an insult devoid of objective meaning.
This board is the only place I have ever seen where the word has been re-defined to refer to people who believe centralized government authority is too powerful, and that individual and economic liberty are in danger of being suppressed.
Whatever other perversions of “fascist” I’ve seen over the years, nothing anywhere else has ever completely inverted the basic concept of a totalitarian all-controlling authority.
If you think this inversion is a “correct” usage, go for it.