This may have already been answered, so you'll have to forgive me, but I note that in City of Broken Idols, the "Advanced Elite Wastrilith Demon" is listed as a CR 15. In the Fiend Folio, however, the plain old run-of-the-mill wastrilith is a CR 17! Can anyone explain the discrepancy to me?
The CRs of most of the Fiend Folio demons were errataed in "Fiendish Codex 1: Hordes of the Abyss", so that they more closely fit with the current CR guidelines (3.0 demons had the nasty tendancy of being over-CRed for their relative challenge, mostly due to the summoning ability).
FC 1 puts the Fiend Folio wastrilith's CR at 11. And, just eyeballing it, this looks mostly fair. 127 hp, AC 25 and a 3d10 breath weapon are sad for CR 17. On the other hand, caster level is high and blasphemy is always a problematic SLA...
I saw your story hour; it looked to me that they would have handily rolled Xerkamat over even though he was supported by many skinwalkers, including casters, had they managed to get a Dimensional Anchor in somewhere. I'm sure they'll prioritize that much higher next time they confront a named demon.
If I remember correctly, the Fiend Folio version was seriously anemic as far as hit points go, and most of the standard wastrilith's normal attacks aren't much to write home about either. If you disregard the potential Blasphemy cheese which doesn't work very well on your players' PCs anyway, they've got their Symbols to cast and then they're basically done for the day. So I don't doubt that CR17 is crazy out of whack for the basic wastrilith.
As far as Khala's link goes, I think that's more part of Khala's CR than Xerkamat. If they fought a fighter buffed by a cleric in one room, and then fought the cleric in the next room, would they get extra xp from the fighter because their enemies cooperated? Probably not; the buffs on the fighter are part of the cleric's CR already. Similarly, they'll eventually face Khala, who uses the link to much greater effect than Xerkamat, and they'll get the xp then.
[Edit: Blasted ninja.]
Thanks for the info guys. This is what I assumed, since the same had been said by the staff in previous posts about other critters from FF. As for my group vs Xerkamat, they actually did have an Anticipate Teleportation in effect, but when the wastrilith actually teleported, the PC in question was not in range to do anything about it. Such are the best laid plans...