First off, nice build. I like the picture frame sump area.
Just a quick suggestion:
It looks like your gate valve is a little high for that ER skimmer, and when the sump is filled the water will be too high for the skimmer. The gate valve really should be on the vertical section of pipe before the "T" and the "T" should be just barely above the point where the skimmer body narrows. The way you have it run is OK (I did the same for my first), but having it on the vertical is better. Remove the metal screw from the gate valve handle and you'll have no trouble with it being submerged (it's the only metal piece on it). As for the water depth, it looks like you'll need to build a shelf for the skimmer. I believe ER recommends between 5"-7" of the skimmer should be under water. You don't have to raise the pump, just the chamber. You can add an extension for the pump to get it up to the height you want.