Well I dont know what your tap water is like.. Is it drinkable? If so, I dont see what 'nasties' can be in it that will affect the ayahuasca brewing...
Regarding mimosa and caapi, I would suggest you do the following: Brew them separately (in the same way, 3x boils, filtering and adding fresh water+vinegar each time). Then, first time you consume, only take the caapi. Drink around 40g.. The ideal is that you find your caapi dosage first, which is when you feel its effects (light-headedness, slight change in perception, etc) but not without having excessive effects (very dizzy, very difficult to walk, etc). So once you find that ideal dosage, next time drink the caapi and the mimosa together (or one just after the other), maybe start with 3g mimosa.
I say this because, lets say for some reason your caapi is very strong, and your mimosa is too weak, or whatever. If you just brew them together, you wont be able to change that afterwards. So better find the potency for your batch and then you can enjoy in the right measure
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Good luck and dont forget to come back and tell us how it went!