That's only for nvidia, it will work.
no. you are wrong. you cant crossfire any cards, they have to be from the same gpu level. like 7870 + 7850, or 7770 + 7750.
even if you could crossfire a 7870 and 7750 it would hinder performance, all the frames would be out of sync and you would get serious stuttering, even more-so than crossfiring 2 o the same card which still gives stuttering on occasion.