For a pump,I'd go with something thats rated at about 650gph at 0'head pressure.Byt the time you overcomer vertical head and plumbing restriction loss.For your baffles/dividers,the dimensions are just ballpark figures.As you place your skimmer/return pump in the sump,feel free to change them as needed.I assumed your HOT skimmer had its powerhead close to the surface.Therefore i recommended a higher divider(10") to isolate it from the rest of the sump.I just measured my 10gal tank,its 11" from the bottom of the glass to the top of the frame.I would go with a 9" divider for the skimmer chamber.You can use 2 instead of 3 baffles after that divider.Go with a 7" baffle useing an under,over configuration.Then with the return pump section,use the typical over,under,over setup.For the float-valve,IME they work best when installed in the horizontal position(parallel to the water surface).Have it drilled in the return pump area where the flaot will sit just under the water level of the last baflle.If installed too low,you'll get the waterfall effect coming over the baffle which will cause air bubbles in the return section.If you need any help/suggestions when building this,I'm often in the chatroom between 9-11:30PM eastern time.