An argument from authority is no sort of fallacy at all. An argument from authority proves nothing, but is evidence for a proposition.

You have a question about car mechanics. You ask two friends of yours - me, a person who has never owned a car; and Mike, a professional car mechanic who rebuilds classic engines as a hobby. You get two different answers. Do you go with my answer, or Mike's?

Maybe one time in a thousand, I'll be right about some car thing and Mike will be wrong. But it'd be a bad idea to bet on that.

There are all sorts of perfectly reasonable arguments like "the argument from authority" that are neither tautologies nor fallacies.

In fact, syllogisms are a trivial and uninteresting part of reasoning. Modus ponens is almost 2500 years old. The real world has very few boolean, binary statements and a whole lot of quantitative statements and subtle distinctions.