"and, I've heard, having no more than one child already in the home, with few exceptions. I'm not sure why, except maybe to mirror it's own one child policy."
Not quite true. You may have up to 4 children under the age of 18 in the household in order to adopt. Also you may only adopt one child at a time, which, IMHO, is a wise requirement for both the child and the parents.
Wow, good to know. Well, in China sibling groups may not need be an issue, but sometimes placing siblings is best for them.