A second DSQC313 will work for the 7th axis. Make sure the serial data out (SDO) of the robot SMB goes to the serial data in (SDI) of the external axis SMB.
You should be able to use some of the wiring from the 1500 cabinet as the S3 & S4 drive circuitry is similar.
The DSQC236C will go in the rightmost (AP7) slot in the drive rack.
The External Axis 4.0 manual in your link is for S4C+. The M94A IRB1400 Product Manual has much of the information you need.