Is a pawn allowed to capture onto a promotion square?
No, this in incorrect. A pawn may capture and promote on the same move, there's no rule prohibiting this (§3.7 of the official rules regulate pawn moves, including captures and promotions).

With two rooks next to each other, the pawn might be 'pinned' because your king is on the same column and there are no other pieces or pawns in between; then, a capture would put your king in check. Otherwise, capturing is just as legal as moving forward.