Re: Water ski length?
Everyone's different, so no real firm answer on anything you asked...more a matter of opinion and preference than hard facts.
I've taught many a 5th and 6th grader how to ski on 68" combo skis (the last two were 90 pound girls). It can be a little challenging for them to keep 'em straight, but they always figure it out despite the fact that these skis are too big for them. My inexpensive combo set I keep on hand for such occasions has a removable bar that connects the two skis together. This bar helps with initial training if their legs are too weak to keep the skis pointed to the sky and parallel while floating. Many combo sets have this feature, but I don't think it's really necessary in your case as your child will already have that part licked by the time he grows into them. If either of these girls I recently taught had previous experience on small trainers I'd assume the 68's would have been much easier for 'em to handle because they would have had the "feel" for what they were trying to do.
I guess if it were my money, and you already have trainers for your 7 year old, I think I'd pass on this set of 64's. If instead you can find a respectable set of 67 or 68's that you and your wife can cut your teeth on, they'll better handle your weight at slower boat speeds when you do choose to drop a ski and start playing with that. Your 7 year old can use the trainers you already have until he hits their max weight limit, at which time I bet he can jump straight to the 68's at that time with no problem. Or, even more likely, he'll be skiing circles around YOU because kids learn much faster than adults! And at that time you'll choose instead to invest in a real nice slalom ski just for him...or wakeboard/kneeboard/wakeskate other such toys he might like much better.
So I'd just buy what you need for now, and don't bother buying a set you might or might not find a use for someday in the future.
Just my opinion, others may vary. Happy shopping!