Parallel ADC IC interface to FIFO Memory
You obviously can't use a 9-bit-wide memory as 12-bits. All you need is to get TWO 9-bit memory chips per channel, one manages the lower 9 bits, and the other IC uses only 3 upper bits, leaving the other 6 unconnected. 

The IDT7203 datasheet shows clearly how to expand the bus to N bits wide, see Figure 15.