Can MkII i/o modules be used in the older Symphony I/O?

No. The newer MkII modules are only supported in the Symphony I/O MkII.

Supported i/o modules for Symphony I/O (original): 

• 8x8 AES/OPT (original)
• 8x8 AES (original)
• 8x8 OPT (original)
• 16x16 Analog In/Out (original)
• 16 Analog In / 16 Digital Out
• 16 Digital In / 16 Analog Out
• 2x6
• 8MicPre 

The only restriction for which i/o modules can be placed in the primary slot or the secondary slot is when using the MicPre module, you must have an 8x8 (original), 16x16 Analog In/Out (original), or 16 Analog In/16 Digital Out module in the primary slot and the MicPre module in the 2nd slot.

To see i/o module compatibility for the newer Symphony I/O MkII, click here.