Rome was definitely imperialist, but i don't think it's fascist necessarily. Fascism requires a lot of stuff that Rome just didn't have. A big one would be like racialized nationalism. Rome was a slaving imperial state yes, but they enslaved anybody regardless of ethnicity. Anyone could be a roman citizen not just italians. Africans, Greeks, Gauls, Arabs, etc.
Its probably not a term that can be used on older nations since a lot of the concepts fascism is built on just hadn't been invented back then. Imperialism is old. Akkadia did it like 4000 years ago. Fascism is pretty new though. It was more of a post-colonial era thing as far as i know.