Don't think you use the blood from centaurs since they are under misc, not aid. Nightstalker blood is used in anti-venom and hydra. As for 12.7mm, you CAN make them, you just need a patch.
Centaur blood isn't used for anything so far, but it's weightless and merchants will buy it. You can make 12.7mm rounds since a recent patch - I did so earlier today, as it happens.