the PCX55 is just for the skimmer. I'm not sure on the dart pump. You may want to email Andy on Myreefcreations.com before going with such a powerful pump.
You can always put a ball valve on the pump and restrict it's flow back to the tank.
If you want to get one pump to do both the return and the skimmer, you'll want to go with something HUGE like a jacuzzi pump. You split the return between the skimmer and the return plumbing.
check out every setup for a large tank you can. Ask what the fish stores are using for pumps. Go see every big tank you can. Take notes. Ask what's worked and what hasn't for each tank.
LiquidBlues, I misunderstood. So you want use the pump as a a return and skimmer pump. I guess it would work but I would imagine it could make dialing in the skimmer a little more difficult. I would get a few opinions (post on RC) before doing this....I'm not sure
As for the sump....Your overflow holes can only spill down X amount of gallons per hr(depending on the size of holes drilled). So if it can only handle 1200gal per hour, any more flow than this would have to be restriced with a ball valve as BC said.
LB, it doesn't pay to go with a pump that does much more than your overflow can handle since you will have to restrict it.
Iwaki pumps are a great choice for their performance and durability.
About to place order.
I just want to make sure that the GEN PCX55 will do as the return pump... it has a rated 1500GPH...
tank is 180G RR, sump going to be under the tank.