It's mostly personal preference and what works best for you.
Last time, I used a bit of vinegar (because it evaporates off while boiling) and it ended up tasting like vinegar. Bleeccch. I have never used acid in the past.
Actually, 6g-25g are all on the low side for caapi. I wouldn't worry too much about overdosing on caapi vine, as the worst that will happen is you'll be puking and shitting at the same time, while tripping harder than you can imagine. I mean, uh, that's a good thing actually... You will be cleaned from the inside out!
But yeah, minimum caapi, FOR ME, has always been 40g, and I usually shoot a lot higher than that. The vine I get is definitely MAOI active at 40-50g, but I cannot attest to anything less than that.
I read once that a good starting ratio is 30g/30g - referring to caapi/chacruna.
How much caapi do you have? The best bet would be to brew all of it together.
You do this by breaking up the vine into strips. You can pound it with a rock, you can stick a knife in there and carefully split the vine by pounding against the blade with something, or you can soak it in water overnight (saving the water of course!) and tear up the now soft caapi into thin strips.
You want it in strips, not chunks. Submerge your caapi in a big pot of water, and get it to boiling. When it's boiling, turn it to the lowest you can without stopping the water from boiling. You can use a lid to avoid water boiling off. Leave it simmering for at least an hour, then change the water and repeat. Do this at least twice, but the more you do it the more thorough the extraction will be - of alks AND of tannins and other compounds.
Combine all extractions, filter through cloth, then concentrate by boiling.
You will want your brew at about 30-50g per shot of liquid. This is easy enough to calculate.
The mimosa is a bit different, as you shouldn't boil it. Throw twice as much mimosa as you need into a bowl and cover it with room temperature water. Rip and tear the bark into strips, letting the water soften it as you go. Squeeze and press the material too, as well as cut it up with scissors. Do this for a long time, and get your friends to help you too. They should be part of this too!
When your water is nicely colored, and you have been ripping and tearing forever, you can filter out the mimosa. Repeat with more water if you wish.
I always concentrate my mimosa tea on the stove, but that most definitely destroys some alkaloids (mostly non-DMT). Make sure you do your math right, and keep it at about 5-10g or less per shot. The extra mimosa you threw in is so you'll have some more ready if one shot isn't enough.
Now you're ready. Keep your brews separate until you know the potency of your plants, then you will know at what ratio to combine them.
Start with a shot or two of caapi, see how you feel. The caapi effect will come on within fifteen to thirty minutes, and is usually felt as a buzzing sensation, an enhanced mood, brightening of lights, mild tracers (higher doses), and usually a gentle, relaxing body high. If you notice this as overt, unmistakable effects, you are probably MAOI'd.
Now you can drink your mimosa tea. Drink your intended dose and wait for effects. Now THIS is the tricky part! Do you drink more mimosa or not? It can come on fast, or it can take its time.
Remember, though, an underdose is a valuable learning experience! Anytime you take ayahuasca, you will go through a lesson, your body and mind will be healed, and you will almost always be a better person for having drunk. Low doses are antidepressant, they can intensify your dreams for weeks afterward, and they really do make your body feel better.
Ayahuasca is an amazing plant, and it can offer you much. You simply must approach it with respect. It's not like it will punish you or give you a bad trip because you didn't have enough respect for it or something, it's just that it takes discipline and proper intention to achieve what one wants with ayahuasca. It's not like a light switch that turns on another dimension like DMT, it takes a little more.