Day Trading

My firm are beginning to make use of technical analysis for trading purposes, and the MACD is one of them. The thing is, the default exponential moving averages (EMA) are 12- and 26-day, which doesn't work for all the stocks I cover. My main questions are, why are 12- and 26-days the defaults, and also, how do you decide which trading day EMA to use when the defaults don't work for a particular stock?

Thanks!!