Whenever I donate blood the actual blood letting part is so fast the nurse makes comments on how it must be a record. Then I ask them if they could use a smaller needle in the future and I get a "no"....
For your veins I recommend tensing your arms afterwards to fully pump and expand your veins to reduce the damage because hardened veins can form from IVs (not common). I still have a vein on the top of my left forearm that's still slightly hard from getting an IV for saline years ago.
As for pin size anything larger than a 27g scares me.
Yeah,that would work well. Just be careful with using the 18 gauge depending on how many times you stick that same vial. If you penetrate that stopper too many times with a big needle you might end up with an area that isn't air tight anymore. I had it once. When you turn the vial upside down to draw from it you will see tiny air bubbles coming up from the stopper, that is the air from the room leaking into the vial. A leak like that could introduce a pathogen.
You can try to move around the point where you penetrate the stopper each time, but after awhile they will start to run together.
When I get my phlebotomies done they use a 23 gauge thank goodness. For awhile I was having to get it done once a month and they realized the scar tissue would be terrible. It takes a lot longer with the smaller gauge but I don't mind that.