Updated.
I can't see budging below 20, compared to other creatures at that CR.
Finished?
Their Advancement looks a bit odd to me:
Advancement #1
Ch’i-lin: 33-42 HD (Gargantuan), 43-84 HD (Colossal)
Duru: 27-36 HD (Gargantuan), 37-50 HD (Colossal)
The female advances to 262.5% of its base HD and the male to 192.3%.
Maybe we should peg both of them at 250%?
Advancement #2
Ch’i-lin: 33-42 HD (Gargantuan), 43-80 HD (Colossal)
Duru: 27-36 HD (Gargantuan), 37-65 HD (Colossal)
I'm also wondering whether we should have both male and female become Colossal at the same HD, since they're basically the same creature, something like:
Advancement #3
Ch’i-lin Advancement: 33-39 HD (Gargantuan), 40-80 HD (Colossal)
Duru Advancement: 27-39 HD (Gargantuan), 40-65 HD (Colossal)
At the moment I'm preferring Advancement #2, but I don't mind #3.
Apart from that I think they're fine.