I cant believe that they made the sump that much smaller.. You have to work with the limited space that you have, the way you have it in the pic look like it will work.
They were not using the filter sock when I got my unit so I dont have one. Every tank is different so its hard to say when it will need cleaning. If it looks dirty clean it, if the water is backing up in the bag higher then it needs to be cleaned. When those thing s get dirty the water does not flow through them very good, you'll know when its time.
is going to be a little low for a return pump. Your pump need to match your overflow rate from the tank. you want as much flow as possible out of the tank, most overflows are rated at 600 gph
. You need to match you return to that flow. You already had a working set up right? Your flow rate should not change just from adding a new sump and skimmer.