There are different editions of Masks and I don't know the differences between them, but in the one I have the cultists have, respectively:
- English 10%, Mythos 3%
- English 55%, Mythos 4%, a "New Yorker cocaine fiend of negligible skills"
- English 1%, Mythos 5%
There's only so much information they could give up even if they wanted to, which they don't. Also, they're fanatics.
They're members of the Cult of the Bloody Tongue, but that doesn't necessarily mean they know what occurred in the ritual of 1916. The oldest of them is 26, so was 17 at the time, and none of them necessarily was even there, they could have joined the cult later or been elsewhere. I would say mythos <=5% means they probably don't know the big picture, but they might be capable of giving away a key identity or two. And the big identity that they might know is in any case spoilered by the title of the book ;-)
They will at least know about the local cult setup in New York, so if the investigators successfully interrogate a prisoner they can learn the relevant location and roughly what to expect inside. That is arguably reward enough. These cultists needn't know anything much about the Carlyle Expedition, so as far as I remember (I've just re-read that scene but not the whole New York chapter) there's still some significant work for the investigators to do. They might well know M'Weru's movements in and out of New York, I can't remember whether learning that now would spoiler the campaign immediately or if it would just help the investigators piece things together later.
However, you should decide the details for yourself -- how much do you want to reward taking captives? Whatever you decide I don't think there's any need to blow the campaign plot wide open. It could be a short-cut through the chapter, or if you want to be stingy you can justify giving away very little. It is extremely difficult to successfully interrogate a cultist for the reasons others have given. I agree that a cyanide capsule is a bit tacky, but even without that how quickly can they get hold of a Swahili-speaking interrogator compared with how quickly can a cultist in captivity do himself a serious and grisly mischief?
Or for top comedy, the first available Swahili-speaker off the roster is also a cultist (on stand-by for just such an eventuality), and helps the captive(s) either conceal information, escape, or die!