I agree that the fertilizer pellets were not a good idea and I am not a fan of Miracle-Gro potting mix. However, the primary problem is that the plants are now in pots that are too large. Regardless of the potting mix used, the oversized pots with excess soil has prevented the soil around the roots from drying out properly.
Wilted leaves occur when the soil gets completely dried out (which I don't think is the case) or if the roots die back from overwatering and are no longer able to absorb water. If your Pothos stays wilted even after the soil is damp, then it is not likely to recover I am sorry to say.
Soil quality can make a difference, but having the right pot size to start with is much more important.
