In other words is the sq6 limited to 12 different and separate mixes excluding the LR?
Yes, 12 is the limit of mixes. But you can route the result of the mixes to any possible outputs of your system. Keep in mind that the mixes can be either mono or stereo, in other words you need 12 (all mixes are mono) or up to 24 (all mixes are stereo) output sockets. And you can send the same mix to several outputs, lets say mix 1 to local out 1 and the same time to the output 1 of your AB168.