You can mix any oil based steroids into the same barrel. Not a problem! I would use more than just the glutes. Even if you are only doing 2 injects per wk, over time, you can build up scar tissue. I would at least add the quads. There are many step by step pictorials online to show you exactly how to hit each site. Here is a good one. http://www.spotinjections.com/index3.htm I would rotate between each glute and each quad. If you are doing 2 injects a wk, you will only need to use each site once EOW. I don't know how much gear you are using, but I will assume it's 2cc test, 2cc deca? If so, then yes, do 1cc of each, 2 times a wk, keeping injections on the same schedule every wk, such as mon and thurs. Delts are also an easy option. 

For glutes, use a 1.5in pin. For Quads and Delts, 1in is good.