Boxers use martial arts training to enhance their abilities, but do wrestlers use Aikido, Judo, and Jiu-Jutsu to enhance their skills?
Who has experience or material on this subject?

I practice Ju Jitsu and I've been practicing with some wrestlers. They told me that it doesn't help them in competition training, but does give them a better insight on what they could expect in a street fight.

Does wrestling have a theory of balance? Do wrestlers use the theory to protect their balance and to unbalance their opponent?

Judo/Jiu-jutsu has a three point theory or triangle theory. Think of an isosceles triangle. (Three equal sides and three equal angles)
... x
... /.\
... /... \
... /... \
... x---------x
If each of your two feet are in a corner of the triangle the remaining corner is the direction to push or pull your opponent to off-balance him or her.

MMA Submission: Ricky Lundell is changing college wrestling - ESPN The Magazine

Not necessarily. It could help with fundamentals, but probably only with take downs. Once on the ground, the rules change dramatically. Almost all holds in Jujitsu become illegal, or potentially dangerous.

Wrestling aids Ju-Jitsu. I fight MMA and training folkstyle wrestling and BJJ. Ju-Jitsu is far too technical in and of itself for a simpler art like wrestling to have an impact on it. Especially since in Ju-Jitsu you can stay on your back all day where as in wrestling, if you're on your back, you lose. Wrestling is the base style for all of these other grappling styles since wrestling came along first, so learning wrestling will help you in learning grappling, but not really the other way around.