In Harvest Moon: Magical Melody(old game, I know :P), how do you get the other villagers to marry each other?

You need the rival girl at four hearts and the guy at less then four hearts to activate a rival scene. But that is all that will ever happen between them. In the English version of Magical Melody the rival marriage option was removed. You can still get the little scene but they won't ever marry. I have never played Shepherd's Crossing so I can't say anything about that game except that I heard it can be pretty confusing. Considering the Harvest Moon games you already have you might want to try Harvest Moon DS/DS Cute it is basically A Wonderful Life characters with Mineral Town play style. If you want something really different you should get Animal Parade (if you have a Wii) it is one of the best Harvest Moon games I have ever played.