I'm not really sure between Merida and Mulan, though both are polar opposites but fun parts of Anna would gel with Merida while gentle/caring/unassuming ones with Mulan.
Aquarious are fun with friends and fight for causes and hates limitations - Merida. They are loyal and intelligent - goes both with Merida and Mulan. Flexibility and adaptability is common between Mulan and Anna. So yea, with either of them for me.
Plus, person wise, Cinderella is way mature than Anna, way more calm and composed.

^ I was thinking the same thing about Cinderella & Anna. I figured Cinderella isn't spunky or outdoorsy enough for Anna. I ended up going with Mulan because she's a little more feminine and open to romance like Anna is whereas I think she might bore Merida with her romance dreams. I figured Merida might be too extreme physically (athletically) for Anna whereas Mulan wasn't a natural at it like Merida and had to train. Also I thought both Mulan & Anna strive to be classier and more sophisticated whereas Merida will strive to fix her mistakes but she's not trying to change herself that much.