i like using 23 gauge needles for venipuncture. my so and i started with 21s because thats the "default" for labs but in retrospect that was silly, where we're going we don't need every blood cell to stay perfectly in tact because its ending up smeared all over my body not in a vacutainer.
exel syringes suck imo. i started with them because shopmedvet didnt ship to my state at the time and they were the only syringes i could find where the store didnt demand a prescription or license from me. they always seem to behave like they weren't pumped even if you totally slammed the plunger, which made blood drawing more stressful than it had to be. i havent gotten any different ones to try yet, though
as for anticoagulant, a concentration of 0.109M sodium citrate is typically used in medical settings. given trisodium citrate dihydrates molar mass of 294.1g/mol, that means 32.0g of trisodium citrate dihydrate is needed per liter of blood.
the huge quantity of sodium in solution resulting from using trisodium citrate seems to irritate my paws. if you have any plans of rubbing blood all over yourself and are using it as anticoagulant bring some hand cream too, it will help a great deal for comfort