Like 11 said, you'll be surprised at how easy, and painless, it is to inject the quads and shoulders. I use a 23g 1" for everything except the glutes (22 1 1/2"), which I rarely inject anymore because of all the scar tissue that has built up.
When injecting the quads, I just do it sitting down, with the leg straight out. It's a breeze. As far as the delts go, I use the front, side and rear heads... although the rear head is a little hard to reach, so I don't do that too often. I do the ticeps too, the rear (long) head.
I personally wouldn't try to inject into the glutes while sitting down, since the muscle will be harder to penetrate. It will probably hurt more... just a guess.
Another thing is, if you inject the quads or delts, you don't even have to put the needle in very far, as long as you are fairly lean. I use a 1", but only go in about 3/4"... even with 2cc injections. But I would go at least that deep.
KB