Are you using this plane as a trainer, or do you already have experience flying other planes?

If you are fairly experienced and really want to fly it with ailerons, then yes, take out most or even all of the dihedral. You might want to leave in a little just for looks.

I haven't built a Senior, but my guess is that there is a plywood dihedral brace inside, and therefore no glass cloth. That's the way my other Sig kits have been.

If you are using it as a trainer, it will be great as is, without ailerons. Plug the rudder into the aileron position. If you have a computer radio, use aileron-rudder mixing so you can work the rudder/nosewheel off the left stick on the ground. That also gives you instant "dual rates" on rudder, by using both sticks together.

Jim