Pet PIRs can be very good, esp. if a single cat I would have no hesitation in recommending two Vanderbilt PIR's - (IR120c or e-line) the only drawback being
both are effectively discontinued due to changes in ownership. I'm sure others can make a recommendation though?
Perimeter protection is a good idea, but needs to be done properly and ideally internal doors should be locked.
Without an expander you will very quickly run out of zones too, as I would personally only fit one device per zone, in the old days it would
be common to have an entire floor on one zone of window/door contacts.