If you fertilize too much, brugmansia's leaves curl into a shape of a boat. That is common when you still have the plant indoors and fertilize. If I were you, I would just water for while. Overwatering causes the edges of leaves turn yellow, sometimes some pests do this as well.

Because I live in Finland I can't tell about miracle potting soil, but almost any normal potting soil would be ok. Brugs are beautiful plants, that are really easy to take care of, you only have to water them enough.

If you use miracle grow potting soil, you probably should not add any other fertilizer until they go outdoors since you have noticed the problem.