Hi RougeR,
Using the Nvidia NVS510 will provide you with 4x Displayport outputs all of which use the 1.2 standard which has support for MST hubs, with this you may use either 2x 4 output or 2x 3 output MST hubs.
This will allow you to output to 8 displays in extended monitor modes.
The mix of resolutions should be no problem and the rotated 21:9 monitor again should not be a problem when setup in extended display mode.
EDIT: Just finished reviewing the monitors, cabling and MST hub you planning to use in more detail, what you have penned down here looks good and should work just fine