I have heard, that merely saying the Three Refuges is enough to declare oneself a Buddhist, if you so wish.
I don't really know the answer to your question. I can maybe say that its common when Asian teachers give the 5 precepts vow to say to the recipients that is okay to only take the ones you feel ready for. So if someone isn't ready to stop drinking, they don't have to take that one.
In general I'm on board with "start where you are" and take an incremental but steady approach in my practice rather than a jump in with both feet style.