Yes, 1600 gallons No more than 5/6 fish, I know they will multiply...any way I can get the same sex to avoid that?
Waterfall or pressurized is really a matter of personal preference.
What is important is that ample surface area is provided for adequate colonization of nitrifying bacteria based on fish load. More (and larger) fish means more required surface area.
Taking into account the way manufacturers label biofilters coupled with pond's being typically overstocked, whatever you decide to use, I would suggest beginning with twice the indicated filtering level for your pond. In other words, Start with filtration levels indicated to support 3200 gallons. This should avoid any real issues in the foreseeable future/
I don't think that any fish can be accurately sexed at the juvenile stage.