Maybe not the advice you are wanting, but a week long extended water fast is not a first step. I would recommend 16-20 hour daily fasts to get used to the process, then move into rolling 48s (44 hour fast, 4 hour eating window). After you get the hang of rolling 48s, bump it up to rolling 72s. After a few of those consecutively, you’ll have a better indicator of your strengths and weaknesses while fasting. These will also give you time to iron out your specific electrolyte needs.
Additionally, I vape THC throughout my fasts, and also smoke cannabis, and have seen no adverse effects (as it pertains to fasting).
As for supplements, I usually save them for my eating window, because I can’t stomach regular ones on an empty stomach, and the gentler gummy ones have too many carbs. If you can stomach them (and they don’t require food for absorption - many do), go for it.
Don’t hesitate to break your fast early if you start feeling unwell. As the other commenter said, maybe don’t try to jump in the deep end first. Try some OMAD and 48 hour fasts before you do a week.
The only supplement you need is electrolytes, and it becomes apparent around hour 36 that you need them if you haven’t been drinking them. My recipe these days is 1.5 parts table salt and 1 part potassium chloride (by volume, I use 1.5 tbsp and 1 tbsp to make about 2 liters of water.)
I would think nicotine sucks for a fast but have no idea. Could definitely make your light headedness worse if you’re prone to that.
In general though, get off the nicotine while you’re improving your body and habits.