Yes i have one that made a big difference. First ~50 shots i did it with classic twisting and there were times i had nasty chest cramps from my pinning hand trying to reach the rear delt.
The greatest way to do it is putting your elbow of the pinning hand against the wall or a door frame to take the tension off your chest. This way you can easily reach your rear delt, no matter how big or stiff you are.
Raise your pinning hand straight at the height of your eyes, close your elbow at 90 degrees and then push your elbow against the wall until you reach your rear delt.
Regarding the injection site, best thing to do as always is to not flex the muscle but if your rear delts are not developed enough it may require to move your arm in a position that rear delt pops out a little bit, it's fine and i pin there either flexed or completely chill. For me it's the less pippy site while side delts for example give me pip for 1 day. I'm doing 1,5ml in the rear delts no problem but obviously i suggest to start lower and don't go full bore, like every virgin muscle.
If your rear delts are not enough developed i'd say to pin with half inch needles, if they have enough muscle you can pin with 1 inch. I prefer 1 inch as i like to go deep.