If I buy bulk beta-alanine and citruline malate, and then cap them (
separately) by myself. What amount of each will I have per cap?
Whats the difference between "0" and "00" caps?

Beta-alanine and CM are both decently dense powders. I would say mix them at around a 2:1 CM to BA ratio, then cap.
I would say in a 00 cap you could get somewhere in the neighborhood of 500mg of CM and 250 BA.
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If I cap CM and BA separately, one capsule would give me 1g of each?
Evolutionary Muse - Inspire to Evolve
BPS - Where Body meets Performance
Flawless Skin Couture - We give you the tools to make you Flawless