chemie,
You have chosen an excellent kit to build. Not only is my Extra my favorite, it was probably the most enjoyable build I have ever done. Now, to answer your questions:

1) None. The extra balsa from the die cut sheets should be enough.
2) I used servos for each elevator half. I also mounted them on the fuse under the horizontal stabilizer. I mixed them in my 9CAP, and am still wanting to experiment with each half operating with ailerons for blurring axial rolls. I did this for counterweight for the Saito 125 GK I have mounted up front. If you do this mod, you might have to get an idea of your balance first.
3) Your 7cap does not provide a mix for dual elevator servos. You will have to get a reversing y harness from Expert or the like.
4) Pull-pull kicks butt. It gives excellent authority to the rudder, and is lighter than a steel pushrod. There are many thread on RCU that go into great detail with pull-pull and 'Ackerman'. Do a search, and you can read about it all day.
5) I can give some better pics of my setup if you want to do this fairly easy mod.
6) Can't answer this one. Have you considered going 4 stroke?


Good luck, and keep us posted on your progress.