You can use a 1/2 in for smaller muscles with little fat on them, like delts, if you have lean delts. You are going to have issues with otehr muscles, like glutes. 1/2 is is just so short though. I'd go with a 1 inch or more in most cases.
Benefit of injecting different sites is that you will avoid the build up of scar tissue. If you just hit glutes every time, you will have problems over time after awhile. Nothing like the "pop" sound of a needle thats ripping through scar tissue as you try to push it into the muscle