Just asking for confirmation from the general consensus here...

So in the most referenced Caapi extraction I've found here, the acidification step is done using 1g of Fumaric Acid to 500ml of H20 ~65g of Caapi vine.

If I want to use citric acid rather than Fumaric, considering the pH differences of the two, 1.6 grams of Citric should do the same trick, yeah? I'm pretty certain, I just want to confirm with others before i go ahead and start the pulling of harmalas.

Presumably you mean the Easy Caapi Vine Alkaloid Extraction Guide?

Citric acid should be just fine, although it has slightly more of an emulsifying action than fumaric acid does. There is no other solvent involved so this should not present any additional problems. I think fumaric was chosen here simply because it is far less volatile than acetic acid so the boiling phase won't stink of vinegar, and fumaric acid has been popular for forming salts with other alkaloids for quite some time now. Citric acid is also largely non-volatile under conditions of boiling in water.

You don't even need to worry about calculating the pH/pKa equivalence, it's just to ensure the water is acidic.

In short: it'll be fine.

Be sure to share details of your results, maybe with some pictures. The ones in the wiki are very nice - wishing you a similar success from over here!