It could be. The salient feature of fascism is malignant nationalism and/or racism. It's generally associated with the right-wing because nationalism tends to rely upon national traditions and glorifies the history of the nation, as well as the old-fashioned ideals of a "glorious" past which conservatives tend to relate positively towards. Because of this, countries under a fascist government have tended to go on a war footing and become militaristic, in which the whole society becomes geared towards war. The public and even the business community might accept this if they are convinced (or perhaps conned) into thinking that the crisis is severe enough as to require such a degree of militarism and "temporary" abridgements of civil liberties. It wouldn't necessarily be socialist or communist, but it might appear that way whenever governments have to contend with what they consider a national emergency or other great crisis requiring the curtailment of civil liberties and possibly even the declaration of martial law.
As for communism, at least in terms of the ideal, it was intended to be internationalistic. For this reason alone, it's fundamentally at odds with the extreme malignant nationalism associated with fascism. In practice, socialist governments have been mostly entrenched and isolated from the world community, and as a result, they adopted a more defensive, quasi-nationalistic posture which took on many of the same characteristics as fascism.
The point is that whatever these ideologies might be on paper or in a dictionary definition, when it actually gets put into practice at the state level, the same mechanisms of politics are at work and a lot of revising and improvising goes on.
