I would NOT mix the batteries! They have different internal resistance and charge rates and will wreak havoc with your controller! You will need a new set of batteries for your PV and solar set-up. All batteries should be of the same voltage, size and preferably manufacture date.
I recently became a dealer for AM Solar (OEM equipment on major motorhomes like Monaco and Country Coach, Prevost and Newell) and we recommend using Lifeline 6 volt golf cart batteries like the GPL-4CT or the new super GPL 6-CT on all new installs that we do. We connect them either in series (2 batteries) or series/parallel (4 batteries).
Further, the controller is actually the most important component in the system, as it is actually the heart of the system. Be sure that your controller uses MPPT (multi-point power technology) to fully utilize the current being generated from your solar or wind sources.