Although late in the debate, to anyone who asks this, I would say:
Smoke detectors on an intruder alarm do not constitute a fire alarm, some householders then say they have a fire alarm when they only have the afore mentioned.
Also most intruder alarms will pulse the external sounder (to indicate fire) and the internal speaker will often make a different tone, but where is the sounder? one is outside, probably not near a bedroom and the other is probably just outside of a cupboard, neither are a high pitch, which means they may not be heard instantly if activated, but a battery smoke detector (interlinked are even better, as is a genuine fire alarm) makes a noise where it is, giving a better warning. So I would say, tempting that it is, do not fit smoke detectors to intruder alarms. Fit either interlinked smoke detectors, or radio linked smoke detectors