Can an HPS ballast be used to power a metal halide bulb?

This is never recommended, the ballast and bulb type (wattage and operation) must match. Convertible ballasts, however, can be switched between MH and HPS modes. There is also the option of using a conversion bulb, this is a special type of bulb which allows an HPS ballast to operate a MH bulb or visa versa.

What is the difference between an electronic ballast and a magnetic core and coil?

Electronic ballasts, also known as "digital", utilize an internal microprocessor that converts a higher efficiency of energy to the bulb using less power from the ballast. The result is a higher quality bulb life, and increased output of light.