Yes, in this lore the Seelie and Unseelie courts were the first two courts in the beginning of faerie creation, made to forever counterbalance each other, so they are the main courts in the faerie realm. The four seasonal courts are not nearly as large or ancient as the Seelie and Unseelie courts. At the beginning, light and darkness were created (Seelie and Unseelie), following that, the light and darkness were split into seasons (Spring, Summer, Autumn, Winter). The Spring and Summer fae could only be accepted into the Unseelie Courts if they declared themselves solitary first - abandoning the safety and protection from their home court, as well as their ability to ever return - and then pledging their allegiance to the Unseelie court, because the Spring and Summer courts are formally aligned with the Seelie Court, as Autumn and Winter are with the Unseelie (once again, the faerie realm is all about perfect counterbalances). I would say that satyrs and sluagh would be acceptable because they are faerie creatures... but the rules I discussed would still apply to them (can't lie, allergic to iron, etc.). Also, with the kith races like satyrs and sluagh, I would ask that you make sure to put them in appropriate Courts. For example: satyrs are passionate and lustful, sounds like the Summer court to me... sluagh are secret keepers who work in the shadows, sounds like the Unseelie court to me.

Also, if this interest check drums up a lot of potential roleplayers, would you be interested in co-GMing? With your knowledge of Changeling and faerie lore, I feel like you would make a good fit and together we could try to come up with a good plot structure. That is, if this roleplay actually takes any root, because I am looking for a LOT of participation. My goal is to have, like, Game of Thrones type numbers of characters, hence why I am allowing and encouraging participants to make more than one charry each. I have high hopes for this roleplay on this site, I have GMed similar ones within this realm on other sites and they fell short for lack of participation or running out of good plot arcs, which I am hoping to avoid with the help of a co-GM or two.