I don't know anything about Minnesota, but I breed tropicals outdoors in St. Louis. I wait till the temp hits 70F, and then move mollies, platies and swordtails outside. The grow and breed like crazy - something about the sun and the algae they constantly eat. The babies need lots of cover in the form of floating plants, to avoid being eaten by parents. When we have an occasional cool night, I just drop in a 100 watt heater, and the tropicals will huddle around it. I didn't get the last of them out until early October this year, and the water temp was down to 56F. Seems they will survive it if acclimated slowly - in fact, I think they are hardier for it. I did give them the heater for the last few days as it cooled off pretty quickly.
Maybe you could start by monitoring your pond temp as soon as the days get warmer and you have enough daily sun to bring the temp up.
HTH
judy