Congrats Alan on your decision to build a Kitfox 7SS.

It will make a wonderful float plane. The 915 should be a great choice for that mission. I'm sure as with new releases of Rotax engines in the past, Kitfox Aircraft will be on the forefront of fitting it to this air frame and designing a comprehensive firewall forward kit for it.

Vertical Power's simplest offering was considered for my build but I ended up unable to justify the price for the less than complicated electrical system I plan to use. Not sure what benefits of running a 24 volt system brings. You may have studied that way more than I.

Paul from Wisc. has discussed in great detail float options and you may want to do a search for float threads and posts. He is our resident float guru and we much appreciate his experience and generosity on our forums. I would love to try a 7SS on floats. Alas, not much water in the desert S.W. for sea planes to play on.
I went with Poly Fiber so no help on the Stewart System. It is gaining in popularity here.

Have you decided on options?
I went with the Laker Leading Edge and the quick build wing options.

Best of luck on your build. Enjoy AirVenture 2016 as I'm sure this one will stand out over all the others for you.
Congrats again Alan.