It's since become a little more peaceful in the tank. The new female MM platy is holding her own quite well. She isn't hiding, and the male isn't chasing as much. When he does chase her, she ducks out of the way. There are lots of leafy plants (silk) to break up the space. The plan is to get a 20g tank sooner than later, and I think the extra space should help with the agression. All four fish are incredibly active swimmers, so I'm sure they'll appreciate a larger space for swimming.