1'' is more than enough, with a 4' drop and use all 1" pipe to the sump you should have a about 1800 gallons running to your sump.
If you use a MAG 7 you might put a gate valve between to slow your drain down for a bit.
As of returns I would use not smaller tha 3/4 pipe, in that case you got max return from your pump.
What do you do for overflow?
As for a skim, it doesn't really matter on this size tank, just what you like the best.
Ofcourse with teeth can be a life saver for small fish.
That my $0.02 for today.