German Journalist Boris Reitschuster, formerly head Journalist for the German magazine Focus' Moscow bureau recently claimed this:
The use of the term "fascism" for National Socialism dates back to Stalin's language regulation in order to distract from the part of the name "socialism". The SED dogmatically enforced this until 1989 in the GDR. And now she is very successful again.
I was wondering, is this true? And if not: when, how and why did the term of fascism start to be applied to the Nazis?
Yes and no.
No, because Stalin did not issue that order to "distract from Socialism". At no point did the Soviets believe that the Nazis were "fellow Socialists". They actually pushed for the prosecution of Hjalmar Shacht at the Nuremberg IMT because as the Finance Minister responsible for the Nazis' economic policy and rearmament, they saw him as the perfect symbol of Nazi "Capitalism".
Yes, because it has to do with East Germany being an important part of the Soviet System. During WW2, Germans were the enemy and needed to be defeated, since the Germans invaded the USSR and launched an unparalleled war of brutality. After WW2, it was important to integrate East Germany - and Germans - into the Eastern Block. The new Soviet Enemies were the West. Accordingly, the Soviets put distance between Germans - specifically east Germans - and the crimes that happened in WW2. Thus, the neutral "umbrella" term Fascist.
This is discussed in more depth in "Through Soviet Jewish eyes", by David Schneer.