To resolve these two issues, you will need to add Intel's USB 3.0 drivers and NVMe support to the Windows 7 (64-bit) DVD.
You will not be able to install Windows 7 without the following solution. But if you choose an NVMe SSD, the Windows 7 installation wizard will not detect it. If you are using a M.2 SSD, it will be recognized by the Windows 7 installation wizard. Then, you should also know that the Intel NUC Skull Canyon supports SSD in M.2 format, but also NVMe SSDs.
Indeed, the Intel NUC Skull Canyon has only USB 3.0 ports and since Windows 7 does not natively support USB 3.0, installation will be impossible from the original DVD of Windows 7. Although the Intel NUC Skull Canyon (NUC6i7KYK) is officially compatible with Windows 7 (64-bit), you will soon realize that its installation will not be so simple.