I think I remember playing C'tis in SP once where I was able to recruit centaur chariots in my cap. It may be rare, but the site should be listed so Zeldor can see it when he looks at the province.
If it's a site, he can recruit as many as he wants. If not, then it's probably a bug, and should not be exploited. Just my opinion, of course.
although hydra hatchlings aren't much of an exploit anyway