You might be injecting into the interstitial space between each muscle group. That could explain any tenderness, but you'd normally feel some serious pressure as it doesn't like oil being put there. Unless you're like 50% body fat (not likely), you shouldn't have enough subcutaneous tissue on top of the muscle to necessitate going with anything longer than a 1" pin imo. Just make sure you're pinning at a 90 degree angle (perpendicular) to your arm and make sure the needle is buried to the luer. You're not wasting the gear either way, so you're probably fine.
You seem like a pretty well informed guy based on your post history, but how the hell do you Z-track your delt? I was seriously thinking about this the other day as I do tend to leak a tiny bit out when I push 3ml at once due to the sheer amount of oil. This isn't an issue in my quads where I can use this technique, but I just can't see doing this solo. Or are you referring to having someone else pin for you using it?