The 'dedicated circuit' for the smoke alarms is an interesting 'catch-22.'
When the NEC began requiring that all the smokes be hard-wired AND communicate with each other, about the only practical way to do that was to have them on the same circuit, and run an extra wire for 'communication.'
The problem was that the IRC - remember how I am always saying you need to know more than just the NEC? - anyway, the 'International Residential Code,' which is the basic 'building code' - specifically forbade such an arrangement, instead specifying that every detector take its' power from a circuit serving the area protected by that detector.
The most recent edition of the IRC does not contain this requirement, so a common circuit should be fine.
Don't forget that 'communication' requirement.