If the market is moving severely against me, what type of slippage can I expect for a small stop loss order on the major currency pairs? E.g. a 9/11 scenario or severe unexpected move by the Fed?
Due to differences in brokers, liquidity, and the size of the event, you will find it very difficult to get an accurate answer to this question. For most events, you can expect it to be reasonable like less than 20 pips. For 9/11-type events, who knows. I don't think anyone can say with any certainty what happens in that scenario.