Would you mix chicken stock with beef bouillon if you were in a pinch ? I though I had enough chicken stock?!?!!

Yes, depending on the dish! What are you making? It might also be possible to substitute water for one of the broths.

Yes, however if you're making something like a creme of mushroom, you'll end up with a darker product.
It's best to keep some crystals or flavor base in the pantry for such emergencies.

Why not just use sherry? Seriously, the recipe I use for French onion soup calls for both stocks. If the French do it, you can too.

I often mix chicken stock with beef broth to cut down the heavy flavor of beef. It is up to your taste. For me, it works fine.

Why not just use water?

I don't know what you are cooking---soup is pretty vague---but I think the answer is yes.