Ed, I am running a barracuda pump on a manifold with 7, 1/2" loc lines, the main manifold is 1". I have to throttle back my barracuda because even with head loss it is too much flow. My tank is 6 foot 180g.
So I think your dart on a manifold plus the stream will be awesome.
I wouldn't use the dart as a return pump, that is asking too much of a single drain line IMO.
I would go with a 500gph pump for a return, I run an eheim, I don't really use it to add flow to the tank, but just to transfer water from the sump to the display etc.
For sump size, are you going under the tank, or in the room?
I would go in the room, and buy a massive skimmer (I am a believer in high skimming, I use an ASM G6). You can use the skimmers you have but I would plan for an upgrade down the road, so plan the sump accordingly.
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