As it turns while the Thermaltake Poseidon ZX does have standard keycap sizes, it appears they use nonstandard spacing for the stabilizers, except the spacebar. I figured this out when my KUL arrived and I tested the keycaps in question on it and they fit perfectly. The vendor seems to understand this and says they will send caps to solve the problem.
By the way, I love the KUL, it feels a little more solid than the Thermaltake, but it is close.