What's the point in just taking a Mandarin exam for the sake of it, if you're already fluent so would probably learn little new from the process?
Also, triple science and double maths is certainly manageable, and i suppose an extra maths would be fine.
But if you wanted to take a humanities subject I would say stick to 4 (max 5 at a stretch) A levels and drop something.
This is coming from an IB student doing 7 subjects (and ToK, CAS, EE) - so I know what academic suicide is. And trust me, sleep and a social life are wonderful things.