First off, how deep can you dig the hole? I can dig 3 feet with my auger. I use a 12 inch auger and it takes several sacks of concrete to fill it back up. While backfiling with dirt is ok for a fence, it's not good for a light pole because it's being pushed and pulled by gravity and the weather in all directions.
A 16 foot treated 6x6 is a heavy chunk of wood. I've installed them by myself, but I was at my physical max walking them up into the hole. If you are really strong, then that wont be a problem. With lights, I've found that the higher up you go, the happier you will be with the results. All of my street lights are 13 feet tall, which is why I would recomend going with the 16 foot 6x6's.
As for twisting, it's a guess which posts will and which ones wont. No way to tell by looking at them. Pick out the straightest, driest ones in the stack and you might get lucky. Personally, I've had much better success with round poles staying straight once installed. For my own lights, I went with 4 inch galvanized pipe. Cost is about a hundred bucks each and you have to use the tractor to stand them up, but once set, they are there forever and they will never twist or bow on you. Mine are painted black, but you don't have to paint them, just leave them the natural silver color of the galvanizing finish.
It's just as easy to use self tapping screws to attach the light to a metal pipe as it is to screw into wood. Almost
You can hide the wiring inside the pipe and then drill a hole in the side to run the wire out to the trench. That makes in nice and clean and you never have to worry about anything hitting the wiring
