So, we know that the Ssanu cast Elder spells and that they have a massive tradition of powerful magics of that sort (ie: Tower of Getulus), and that these traditions were (according to myth) taught to them by the Dragon offspring of Kassagore and Yig. We know that the Ss'ressen can NOT cast Elder Spells, and instead use Eldritch magic in the rare instances (leaving Ven's brood aside, or Shamans of Jeggal Sag) that they can cast Arcane magics, and that Eldritch magic is less of a learned skill and more of a 'felt' way of manipulating the arcanum (I compare it to Elder being writing an Orchestral piece, and Eldritch to free-style jazz).

With how different the two types are, were Ss'ressen allowed to enter the Black Coil?

I always took Eldritch to be more the 'learned skill' and Elder to be the 'felt way of manipulation'. Eldritch is based upon Logic and there are no backgrounds to start with Eldritch Casting. Seems many Eldritch casters have masters/apprentices, which leads to a learned skill. Obviously you need the 'spark' to be able to do it, but I've always seen it as the more taught type magic.

John