Opening principles and guidelines for pawn structure are things you should pay attention to, but you can break them if you have a reason.

In the Stonewall, you do have the backwards pawn and you do weaken your king’s protection, but it’s ok because the opening is designed to lock down the middle and rotate all your pieces for the kingside attack.

You can be reasonably comfortable (if you learn the lines and responses to opponent’s lines) that your king will be fine.

That backwards pawn also is well-protected and serves two purposes:

1. It protects your pawn chain from getting undermined by a c-file pawn push, and

2. When opponent doesn’t try to undermine your structure, it allows you to push it forward to break the opponent pawn chain in a way that advances your attack.

Bottom line: rules are important, but you always have to consider whether they apply to the game you find yourself in or the opening you choose.