I've heard that mixins can be unmasked if outputs are spent with mixin=0. Is this true?

(From tacotime) It refers to a well-known privacy issue on CN coins. We've had a fix for this in the development for weeks now after corresponding with a Bitcoin core devs that should be more effective than the solution for Boolberry, we're just waiting until the core of the software is more mature before we roll it out.

There are no "coins" in your wallet, there are "outputs" corresponding to transactions you have sent or received. These outputs can only be spent in their entirety. So if you have an output of 10 that you received and you use it to send 1, then a transaction of 9 is sent back to you as change. As with any transaction, this one coming back to you takes time to confirm. Until that time the wallet reports that 9 of your balance as locked.