subfloor thickness etc.

Hello, I am looking to install about 400 square feet of 3inch by 3/4 tongue and groove unfinished oak flooring. My subfloor is 19/32 plywood. This I guess meets the minimum requirement of 5/8 but I am thinking of installing 3/8 plywood over the existing plywood floor. My question is that the subfloor is visible from the basement and it says on it leave 1/8 gaps at ends of sheets. Do I ignore this and just lay the new plywood underlayment right over this (even overlapping the seems) or do I need to try and cut the new plywood to exactly fit over the old (seems impossible)? I'm just looking to firm up the floor as when the kids jump etc the cd player skips. I guess I could skip it and try to hit joists when nailing the flooring down? My second question is I know its easy to buy the transitions if I don't beef up the subfloor but can I buy them if I raise the subfloor up by this 3/8 underlayment ?(it will meet 3 bedrooms with just carpet and will be quite a bit higher). many thanks folks!