How much sugar should be used for the compote? And should it be plain sugar?

Start with a teaspoon or two of sugar (based on similar recipes, and the volume of regular plums in this recipe). Cook, taste & add more if you want.
The headnote sees the miso sugar as a replacement for regular, so you could use that.
Or, if you want to ease into use the miso flavor, use regular in the compote and miso sugar for dusting the finished dish just before presentation.