My understanding is that the AC12 is in a half-width 1U sized box and that two of them can be mounted together in a 1U rack space. Four of them would be required for 7.1 DSP speakers with SPDIF/COMMS connections, so a 2U rack space would be required for a full DSP setup.
I have heard that it is best to rewire with the speakerlink cable to the first of every speaker pair, placing the AC12 behind the first slave speaker (as a backpack) before passing to the second with the existing meridian digital speaker cable. This maximizes the runs of balanced digital audio from processor to speaker and minimizes potential interference.