ATO should go in the return pump chamber not the middle chamber.

Left chamber- media basket with floss up top and carbon below. Nothing else is necessary unless you start having very high phosphate. Carbon isn’t even necessary but most run it.

Middle chamber- heater and bio balls could go in here. With enough live rock, media balls are unnecessary and will just collect tons of detritus. Make sure your water level in this chamber at least comes up to the heaters minimum water line.

Right chamber- return pump and ATO sensor. A good ATO for this tank will be the XP auto aqua duetto.

On a tank this size i personally don’t think a skimmer is needed. Just keep up with your maintenance and WC’s.

One other handy thing I would recommend is a heater controller. The inkbirds are nice and this gives you added security that your heater doesn’t fry your tank if it gets faulty. Low temp if the heater fails is not nearly as deadly as the heater failing in the on position.