All are standard IDC connectors (except one micromatch*, which still uses the same ribbon), so yes, 1.27mm (50 mil) is correct for the ribbon wire.
The BOM calls for 2 feet each of 10-, 16- and 24-wire ribbon (6 feet/ca. 1.8m). In theory, it would do to order 4 feet of 26 and split accordingly. Note that you’ll miss the red wire this way! Or just simply grab 6 feet of 26-wire and have some to spare.
*MPE 371-1-010 is a part on Reichelt, seeing as the Mouser part will be out of stock for a few months more.
Also just replaced the (very standard!) USB B socket that was also listed as out of stock.