If you put a Skink chief on a Stegadon, does it lose any of it's special rules?
Nope, it doesn't.
If you put the Skink chief on a Terradon does it lose it's Drop rocks rule?
Frankly speaking, I doubt it would.
In my opinion it's the Terradon that carries the rock and I doubt that the Skink chief considers his Terradon too good to carry a rock so he can better crush the enemies of Lustria...
Cheers!