There have been more white slaves throughout history than black ones. I'm sure American slaveholders would have accepted any race. Africans just happened to be fashionable at that point in history.
Depends how you define "history."
I'm sure American slaveholders would have accepted any race.
Earlier maybe, but not so much in the 19th century. The country was high on liberty and democracy. Whites weren't going to deny rights to Whites, so they had to use another group as slaves. Ironically, what we call "racism" may have become such an important factor because slave owners started to have scruples about who they could enslave.