I'm using 25 gauge, 1"ers and find them difficult to fill. It just takes a little time for the stuff to fill the syringe.
When I was using testosterone treatment, my doctor told me to use one syringe for loading the med, and then switch out the needle to a smaller one for the actual injection. I initially had a prescription for the syringes, but found buying them online to be less expensive. The syringe I used for drawing in the dosage was a BD 3ml syringe, luer-lock tip 18 gauge x 1.5 (1.2mm x 40mm). My wife would then substitute the larger gauge needle with an Easy Touch 23 gauge 1" (25mm) needle. I'm sure a 25 gauge would have worked. The needles simply screw off and on so they are easy to change out. I was told by multiple sources (doctor, nurse, phys. asst) to rotate injection points every shot so one week would be in the right butt cheek and then the left the following week. If self injecting, I was told to do it in the thigh muscle, but also to rotate from thigh to thigh.