1.) Its schedule 80! Good luck, its a pain in the rear to find. You can get SCH40 that is close, and sand it down with 80 grit, lots of glue, luck and time. Or find a store that has SCH80 (again, good luck, try places that run HUGE sprinkler systems).
2.) Feeding from an overflow is PRIME! But only if you can have that one clogged without a flood. I tried it, and it was way to touchy for me. A MJ1200 is not enough flow in my opinion. My Octo skimmer is fed by a mag 7.
3.) Trial and error works hehe.