A problem might be that if you allow both Pawn and Berolina as promotion choice, you don't only allow Berolina to promote to Pawn and vice versa, but you also allow Berolina and Pawn to stay themselves. Since the choice would make no sense on the last rank, where neither of these would have any moves, I suppose this is not a problem, but rather exactly what you want.
I don't see the way to ban promotion of regular pawns to berolina pawns and vice versa.
I don't see the way to ban promotion of regular pawns to berolina pawns and vice versa.