Using FPL (18/4 ) cable for 120v Smoke detectors
You absolutely cannot use that cable for line voltage smoke detectors, for several reasons. One big one is that you cannot have a typical circuit smaller than 15A, which would require #14cu minimum. OK, I have to say, it is technically possible, but entirely impractical.

IMO replacing "alarm system" heads with line voltage smoke detectors is a downgrade and rather hackish.

Just how big is this house that you have 18(!!) smokes??