The IR sensors will be wired to your controller with a zone monitoring input, correct? I would suggest pointing the IR detector towards the point of entry like a door or window. Another thought, instead of using your low-level PIR for pet detection, just use the high-PIR for intruder detection (Unless your concerned about low crawlers.
). You can mask off sections of the field of view of the PIR to focus specifically on door ways or other areas of the room.
You might consider using reed switches to monitor the doors and windows. This would typically be done with a resistor in parallel with the switch at the end of the line(EOL), screwed across the switch terminals to monitor the normal condition (what ever the resistor value is). When someone breaks in the switch shorts the resistor and the controller senses the alarm condition.
This would also allow you to use your PIRs in a logic AND fashion, it that the PIR trip will be accepted only if another zone is tripped. It will also increase your coverage, since the doors and windows would be monitored, the PIR could monitor the center of the room.
Glass break detectors are another option. These can sense different types of glass breakage, such as normal pane glass or tempered glass. These should also filter out pet noise, such as a dogs chain when it scampers across the floor.
Also, you might consider using smoke detectors, either silent or with a piezo.
You can use a silent smoke if you use an alarm bell. The bell can be used for both burg and fire. If you go with a silent smoke, make sure they can be monitored by your controller (i.e. you can strap an EOL resistor across it).
Will this report to a central station or just be a local alarm? You could use networking to communicate as well. It just depends on how advanced you want to make it.
Flashing lights are a good idea. They draw attention.
You might also want to use relay/open-collector outputs to drive lighting or other things.
There are also mfg who build wireless zone monitoring equipment. This save on the install.
You will want a backup battery in case of loss of AC.