I agree, they shouldn't speak.
+4 turn resistance in tomb-tainted?
I'm inclined to go with greater and elder, also, just so there's more spread for the spell levels.
Updated.
I question the inability to at least understand a language. The elders are as smart as an average human, and the "dumbest" among them are Int 4, which usually allows for at least rudimentary language.
Shall we work out the spell levels of healing spells to "summon" them?
Looking at the summon monster/nature's ally lists:
3rd/2nd: Small
5th/4th: Medium
6th/5th: Large
7th/6th: Huge
8th/7th: Greater
9th/9th: Elder
So maybe...
0-1st: None
2nd-3rd: Small
4th: Medium
5th: Large
6th: Huge
7th: Greater
8th-9th: Elder
Epic: Monolith
I think of it more that their thoughts cannot be parsed by entities who are not native to the Negative Material Plane.
Similar to the Xeg-Yi, which "have high intelligence, but their minds are alien and their thoughts are incomprehensible to humans."
I'm thinking we might be better off setting it to the higher level of the lists, e.g.:
3rd: Small
5th: Medium
6th: Large
7th: Huge
8th: Greater
9th: Elder
I want 4th-level in there, so there's not gap. We can start with 4th, or simply have a range at one of the levels. I'm not much bothered either way, as long as we don't leave off 4th.
I'm still strongly anti-gap.
Let's await freyar's take.