Your system is sort of "maxed out". Anything you add now makes things confusing and costs money, which could and should be spent in a better solution. What you miss is auxes for in ear monitoring. Mono in ear is NO option for in ear, so you need 12 auxes (6 stereo) at the moment, maybe more some time later. If you want to stay with PreSonus and really need 32 input channels (besides the Aux Ins), a Series III 32 or 32S would be a solution. The 32SX (24 input channels) is also interesting, because it has 16 physical aux outputs.
Otherwise you could think about using the "Pre-Insert Balanced Direct Outputs" (1-8 and 9-16) on the back of your (Master) mixer. It may be that you can also use these outputs of the cascaded mixer. In this scenario a a third "monitor" mixer would be needed with at least 12 (or 6) auxes to feed the headphones. This article from Knowledge Base might be give you some ideas:
http://www.presonus.com/learn/technical-articles/Earmix-Personal-Monitoring-System-Application-Guide