I suggest that you put a layer of 10 oz glass set in epoxy over the ply, particularly if you use fir marine ply. That will eliminate checking and give you a much more robust and waterproof surface. I would glass over the sole too before you lay the teak. As far as the teak sole goes, you might want to build it out of the boat and then glue it down. Our local builders that build their own teak cockpit soles all do it that way. Much easier on the lower back to build the sole at waist height on a bench than in the cockpit.