I agree that a good trading system is important but you also need to have the trading psychology sorted. If you don't have the nerve to hold trades open long enough you will never make a profit.
I trade just EURUSD. I agree that it is best to trade just the one market, especially when you are starting trading. A really important part of trading is knowing how a market behaves and each currency behaves differently. You need to focus on one market before considering to trade others.