I mix em up all the time. Its probably kind of bad practice but sometimes I like them better or they are the only type with a color I need. I have no problems painting over them in small areas. If you try and paint a enamel over a large area of acrylic you may start to get an effect along the lines of trying to color plastic with a Crayola marker. Use 2-3 thin coats to get around that. I have not had my acrylics be eaten by my enamles either.
I do much prefer enamels though. They take longer to dry completely but I don't need to deal with them pealing and the sort.