But in the GECK, it says that the chinese stealth armor only occupies the "upper body" slot...so why cant i wear headgear with it?

You should be able to, it's probably glitching...try reloading a save before you got it. Also, make sure it's the right chinese stealth armor

try putting it on and if it doesn't allow head gear, take it off and put on head gear then put on the suit. sometimes that works for me.

you should be able to wear as many hats and helmets as you want when wearing the suit. there is an exploit involving that suit. maybe this is just another side effect from the suit