They hold a fixed amount you can draw safely, just like your own potential is a fixed amount you can (eventually) draw safely. But just like an unaided channeler can overdraw, when you use a sa'angreal that doesn't have a buffer, you are now overdrawing. I don't see what's contradicting the sa'angreal being a pool of additive power, here.

I thought of it a little differently. I think that your own potential is basically how much saidar you can draw on your own without damaging your connection to the Source, which is like a tube (conduit). When you use an angreal, it is like a link (as Elayne tells Aviendha, even the way you use an angreal is like linking with another), but a link to an inanimate object that holds someones potential, or a percentage of their potential.

Not being a human being, an angreal won't draw the Power on its own, but its like an inanimate channeler, which is why channelers can sense angreal and sa'angreal nearby.

But one thing is different. In a typical link, you're drawing a little less than what you and your partner can draw. This is the buffer. It prevents you from reaching the ceiling of your ability and damaging your conduit or your partners.

Angreal, being inanimate objects with no actual potential, don't have a real upper limit. So unless they specifically make a buffer for it, you can link to an angreal and potentially overdraw. We know from RJ's statement on Callandor that adding a buffer is advanced work. I think once someone's potential (or many people's with sa'angreal) are invested in the object which is to become an angreal, you make some advanced modifications that prevent it from being able to draw anymore.

The reason I think the CK can only be used by people above a certain strength is that if you have a greater potential, your soul is better able to deal with massive amounts of the OP flowing through. We know stronger channelers tire slower, for the most part. So when a new conduit is plugged into their soul in the form of an angreal/sa'angreal, they are better able to deal with the increased exhaustion that comes with using all that OP. For most sa'angreal and angreal, this just means that you give the stronger ones to the stronger channeler. But the CK are SO powerful, by several orders of magnitude compared even to Callandor, that the exhaustion will quickly kill some weaker channelers. You need to be at least as strong as Siuan, and preferably much stronger to be able to have that amount of the OP flow into you and not immediately die of exhaustion.

Here's a clue from Cadsuane that hints at this:

For Nynaeve, several hours of that flow wasn't enough to kill her. For someone weaker than Siuan, it might be a few minutes, or even less.

Sure, but for myself, I'm perfectly satisfied that it is a pool of additive power. The only wrinkle is Elayne's "weak angreal" statement, but that isn't much of a one, for me.