Why the 8000? I know that it has a larger pump than the 6000/5000. My question in in regards to rating. The 8000 is rated for tanks up to 1000 gal. OP has stated that total system volume is going to be approximately 600 gallons. Wouldn't a 6000xp SSS (rated up to 640 gallons heavily stocked) draw less power, take up less space, cost half as much as the 8000, and meet the skimming needs of the system?
My thoughts are that bio load might not be high enough in the system to produce a consistent head with the 8000. Just my $.02.