No there is not a higher probability of hitting vein in shoulder than quad actually I would say the opposite. Just make sure you inject in the proper location.
www.spotinjections.com . It sounds like you should avoid quad injects with a 1/2 needle. This happened to me actually last week. I am fairly lean and I use a 29G 1/2 in pin. One in the delt and one in the quad. The delt was fine. The quad got red after a day or so because a lot of the oil didn't make it into the muscle. It did not get infected or anything jsut irritated. Stick with smaller muscles for 1/2in biceps, tricep, delt . The delt would be best. The needles are so small that they will not cause scar tissue which means you could use your delts almost every time you inject. You don't have to aspirate. They have changed that rule recently. I still do but it is not needed because there is not a significant risk of htting any veins in proper sites when done correctly.