With a dryad, I'd say something with trees, indeed. Perhaps a particular family of trees, as in the myths dryads are often bound to a certain tree, so he/she could only shift through pine trees, for example.
Red Court Infected: I wouldn't let them have that shifting ability at all. I'd say the key is there is a place attuned to the nature of a supernatural creature, so the creature should embrace said nature. RCI's don't exactly embrace theirs. Thomas, while reluctant in some ways, still embraces his WCV nature, so he could shift in and out at certain places.
In the case of changelings, there is little difficulty: either they accept their faerie half, or renounce it. A RCI has this state inflicted upon them and if they accept/embrace it, they will turn into a RCV, but until they do, I doubt they can shift into the Nevernever without a special power/item or help.