Its not used for adult females its only used for little girls or it could also mean daughter and if it is used as daughter then you can use it for an adult female. Same logic applies to boys
I think it is something like this:
waladun Arabiyya --> Arab boy. Also, waladun dhakiyyun --> smart boy. Then for consistency:
Amrikiyyatun suriyyah --> Syrian American.
This is a tricky question. Either order sounds fine in English, but the sentence would have to be "Rosa is an American girl." In English, we use "an" instead of "a" when the next word begins with a vowel.