What is the difference between Square D and QP single pole circuit breakers, and can they be mixed?
Some research says that a QP circuit breaker is the 1" or 2" wide breakers that protect a 110V or 220V circuit respectively, and is made by Siemens.  Square D is another manufacturer of circuit breakers and service panels.  

Ideally, you would want to have all your circuit breakers be manufactured by the same company that makes your service panel; the manufacturer will -- one hopes -- certify that their breakers will work correctly in their panels.  Some manufacturers produce breakers with the same mechanical fit as others, in which case it would be up to that manufacturer to say if their breakers will work in a panel made by the other.

I'm assuming that the Square D breaker actually fits in the service panel, so to determine if it's OK, you'd have to do some research to see if Square D certifies their products to work with the panel, or if the panel's manufacturer does the same.

(Side note: per NEC2008, smoke detector circuits have to be protected with an AFCI if there's a detector in a bedroom.  This might explain why that circuit is different.)