A few observations (from a relative newbie)...
1) Do you plan to have a sandbed in the refugium? You might need to raise up the spraybar.
2) A pre-filter sponge on the return pump might trap 'pods
from the refugium and prevent them from getting up to the main tank.
3) Regarding the drain, here's what I encountered while recently plumbing my sump. First, having the drain extend deep into the skimmer section caused problems for me. The overflow (the standard Durso-like that comes with an All-Glass reef-ready) would regularly make a "toilet flush" sound, which I eventually learned was due to air that had been pulled down into the drain pipe along with the water, and was trying to fight its way back up, causing drain efficiency problems. Cutting my drain pipe to just extend an inch or two below the surface solved that problem. However, it caused (or simply revealed) another problem... a very obnoxious "pot of boiling water" effect in the sump and a lot of bubbles. It was suggested to me that the way to fix this was to replace the pair of 45 elbows that made the last turn down towards the sump with a tee on its side so that one end is open and faces upwards, and the other end goes down into the sump. This gives the previously-mentioned trapped air an easy way out of the system. A larger final standpipe into the sump was also recommended. I used a 1 x 1.5 x 1.5 tee (my drain pipe size is 1 inch), and also ended up placing a short section of pipe along with an elbow with some floss stuffed in it (to muffle the drain noise) to the open end of the tee. Here's a photo (the drain is partially obscured by the skimmer)...
I've also been battling microbubbles being returned to the main tank. With some experimentation, I determined that both the skimmer output and the drain from the main tank were introducing a lot of bubbles into the sump, and enough of them to cause problems made it back to the main tank (despite a bubble trap after the skimmer section).
I rotated the skimmer to have it's output be further away from the first baffle, and I also modified the end of the drain pipe. I added a pair of 90 elbows and a short section of pipe so that instead of discharging downward into the sump, it discharges upwards, with the final pipe extending about an inch above the surface.
Forcing the water to the surface like this, along with the common-sense skimmer rotation, effectively eliminated my bubble troubles.
Now, that's not to say that YOU will have to go through all this... it's just what I ended up having to do.