Question:  if my trading broker goes bankrupt, can they use my money to pay off the debts? like in MF Global?

Answer:  When a company goes bankrupt, there's a pecking order the company must go through to liquidate their assets.  The bankruptcy laws usually help dictate where goes where, but it goes pretty much in this order from first to last:  special creditors, secured creditors, general creditors, stockholders (of tradeking.)  

The stocks/securities you own in your account are insured by SIPC.  SIPC:FDIC = Financial Institutions:Commercial Banks.  If TradeKing is anything parallel to Madoff, then you will receive nothing.