Morgoth did not create Sauron. Sauron was Maiar, a powerful demi-god, much more powerful than the balrogs, and became Morgoth's deputy. Morgoth was not defeated by the elves or men - it took 2 invasions from Valinor led by gods and demi-gods (including Olorin) to defeat him. Balrogs were not all of the same capability or strength, though they were all what may be collectively termed fire elementals. Not all elves are of the same power either - how could you compare Lady Galadriel with a rank and file elven soldier? The elves managed to defeat only a few balrogs - it was the coming of the Valar and their vast armies that defeated almost all of the balrogs - only DurinsBane remained in the record, hidden in the depths under the Misty Mountains.
For what you have said, I feel safe in assuming you have not actually read The Silmarillion or any of the background tales of Middle-earth. You can get a lot of this history and information in useful cross-indexed format on these two sites:
http://www.glyphweb.com/ARDA/
http://www.tuckborough.net/
Morgoth is banished to the Outer Darkness for eternity. Sauron may have wanted dominion over Middle-earth for himself, or he may have wanted to use that power to 'rescue' his master.