Can Skybreakers vow to follow a code of rules some might consider outlaw-ish, like the Pirate Code. Are they obliged to adhere to changes in the law after their vow?
Yes and yes.
What you're running into with what's happening right now, the Skybreakers are under the thumb of someone who has a much more rigid interpretation of what they should do than is necessary for the Order. And so you could totally be a Skybreaker who is not of this group, and this group would not look kindly on something like the Pirate Code necessarily. (Though the Pirate Code kind of works for them, because it's in international waters, so even with the current crop of Skybreakers you could probably argue the Pirate Code, and they'd probably be okay with it.)
But you could have even less, codes that's like, "I'm going to follow the code of the criminal underground. I'm going to follow the Mafia code." Current crop of Skybreakers, that would not fly with them. But in the Order in general, and the way that highspren work, and things like that, you would totally be okay.
Which is kind of dangerous, yes. But you would have to follow the code as the code changes. So that could get you into trouble, also. Skybreakers, they've got an interesting way of going about all this. Hopefully, all the Orders do; that's one of my goals with them.