I'd say the twin skimmers would be plenty and the second could be added later, no need to rush out and blow the $800 now. There's plenty of big salt tanks out there running no skimmer so I'm not going to say it's an absolute requirement but something to consider none the less as it would be quite beneficial. It also sounds like it will be cheaper to buy two skimmers rather than one larger one, plus if you every sold them you'd do much better with the smaller models.
For the sump volume just make sure nothing bad happens when you cut the power. Might be fine but to me looks like it runs around 2/3rds full so you have around 23 gallons of space at an absolute max. 1 inch from the display is 21 gallons so when you add the other factors you're short for sure. I also usually budget 2" from the display to be conservative as the other-flow teeth can clog up with algae and raise the water level a bit plus siphon back-flow will usually drop the water level in the display below the overflow. Check valves help but I don't recommend relying on them.
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