Always aspirate for intramuscular injections. You even aspirate when you're giving horses injections. Standard medical/veterinary procedure.
I used a 23 gauge too, and yes - it takes ages for the T to drip into the syringe. Typically hospitals will use 18 gauge to draw it, so they don't waste time, and then switch to a slightly higher gauge for injections (20?). (smaller gauge = wider needle) My prescribing doctor recommended a 25 gauge, I guess since I'm a teen and he didn't want me be scared of a big needle while self injecting... my dad overruled it, said a higher gauge would just be more difficult to work with.
Typically in hospital situations testosterone is injected in the buttocks, probably why the pharmacist was confused. Obviously, for self injections, you have to switch locations.