By the way Senior, I was thinking about your huge stir plate that is too strong for a 1-4L starter and I have an idea to make it the best stir plate around...
Simply put, add another switch and resistor for a separate circuit. The switch switches between the current stir plate circuit and a step down circuit that cuts your voltage in half. Label the switch Small and Large. Voila. You should be able to put a 500ml starter on it on the "small" circuit, and a 5GAL starter on the "large" circuit".