All of the smoke detectors I've seen specify a dedicated circuit. They also require them to be interconnected so if one goes off, it trips the others. This interconnection circuitry usually requires the detectors to be from the same manufacturer, but some do offer an extra cost compatibility adapter. The industry standard recommendation is to replace the smoke detectors every 10-years, so it might be time to do them all in the house. Recent changes to the codes require more detectors than previous code releases...you'd need to find out what code your town is currently using. You could always do it per the newest set of code. Don't quote me, but I think it now requires one in each bedroom and at least one on each floor, all interconnected and a/c powered (so people can't steal the batteries and make the non-functional!). My personal preference is to also have battery backup and a momentary alarm suppression button. The time you may need them most is possibly during a lightning storm when the power is out...a fixable deficiency in a/c powered detectors (add a battery backup!). Many allow you to use a lithium battery that could last the entire 10-years without ever having to change it out (depends on how often and long power outages are).
The bathroom requires a dedicated 20A circuit, and might need more than one depending on what you have planned for it.
A home theater room could be drawing a lot of power...I think you'd want more than one circuit there as well. Your panel may not have enough spare slots...time to bite the bullet and either add a subpanel, or replace with a larger panel.