I have only ever injected into my quad. I chose the quad as the first place to go because it seemed like the easiest place to aspirate (pull back and make sure you don't get blood before you inject). And after that I just flowed the "if it ain't broke..." logic. If I pin twice a week, the mark from each pin goes away in a week. So I only ever see two.
The emphasis on rotating is if one is really prone to scar tissue or if one is using a lot of short esters that need to be injected daily or every other day. Or I suppose if you wanted to rotate post injection pain areas. And then some guys believe that the muscle injected into will get disproportionate benefits...
Get something like an 18 to draw with and switch needles to like a 23 or 25 to inject. Online suppliers of bulk needles will give package deals of one rig with two pins. Sometimes a 25 can be hard to push down and the tendency is to pull the pin out a bit as you push down, so try and keep the pin in deep the whole time. Only the initial prick hurts a bit. If you consistently see a bit of oil leaking out, just load an extra. 05cc if you really want to be neurotic about it.
Post injection pain varies over the compound and the ugl or brand you use. The test I have now has no pip at all, but the masteron I had earlier this year have me a couple painful shots. Oddly enough, it never hurt in the gym, but only doing things in ordinary life.