Two points, just for clarification:
Sidhe is a special category *in addition* to whatever type of fae you already are. So there are pixie sidhe, troll sidhe, unicorn sidhe, etc. Some sidhe are so far beyond the base type of fae that they're hardly recognizable as fae at all (Lea, for example), but others are 'merely' stronger, better, nobler versions of the base creature.
Also, remember that Seelie Magic and Unseelie Magic require at least some degree of approval from one of the Queens to be used at all. A changeling can always have their racial powers (e.g. strength for a troll's son), but they should only have Seelie/Unseelie magic if one of the Queens believe it could be to their court's advantage to allow the power to be used by that person.