Building the Quick Canoe

Hi all,
I have chosen my next project; to build a Quick Canoe. My wife doesn't quite know that I'm planning this, but she will when the package from Bote Cote arrives on the doorstep tomorrow :o.

I have a few semi-questions about the project that I hope people can help me with.

1. I am thinking of building it using 6mm ply for the bottom and 4mm ply for the sides to save a bit of weight. Cost-wise it will be about the same price as making it out of all 6mm. Do people see it as a real issue?
2. I'm going to build bouyancy tanks fore and aft, partly for safety, partly for storage. What's a good length to make them? I was going to play it by ear after putting the seats in and seeing where my legs came to, but was thinking that 500mm or 600mm would be a reasonable length.
3. My intention (once my wife sees the merit and beauty of having a canoe) is to "upgrade" by building the outriggers and a sail for the canoe. I was thinking of putting a sternpost (or whatever you call a reinforcing piece of wood at the back where the two sides of the hull meet) in, so that I have something to attach to if I want to add a rudder to it. Is that a good idea?
4. I have 3 young kids who I know (based on prior experience messing about in canoes) will all want to come along in it at the same time. If I replace the central spreader with a middle seat, will that unduly upset the canoe's structural rigidity? I think I saw a QC that someone built in central Europe that had done this, but just want to check peoples' thoughts.
5. It will mostly be for use on the Yarra (Heidelberg to Kew Boat House) and for car camping trips to places with lakes or slow-moving rivers. Will the skegs (as per the plans) be too "grippy" for river use?

I think that's about it. The plan is to build it in the upcoming school holidays and have it on the water by mid-October (famous last words, I know - but we're off to Anglesea in the December holidays and I need to have it ready for use by then).

Cheers, Mike