Hi, Welcome to the forum.
First of all, the term PI (Pet Immune) is a misnomer. (In this instance, "misnomer" is a euphemism for "lie".) Some of the better PIRs are pet-resistant, but none are pet-immune. If you have a pet, and especially if you have more than one, you are susceptible to pet-caused false alarms. The “PI” motions will reduce the number/frequency of alarms, and in some instances will actually eliminate them, but anyone who tells you they are actually pet-immune is not telling you the truth. They’re passing on wishful thinking.
As to “PI” PIRs rated up to 80 pounds---folks, I personally know someone who did a few burglaries when he was a skinny15 year old kid and weighed less than 80 pounds. Not all burglars fit the profile of the sales hype, husky guys with a crowbars wearing masks. In fact, a lot of burglaries are committed by teens who live near their victims.
The PIRs don’t measure the weight of critters moving in its field of view. An active playful 20-pound cat chasing flies across furniture is more prone to trip it than a lumbering 12-year-old 80-pound mastiff. But sometimes even the old dogs get excited and go after a passing moth.
Having said that, if you want to take the chance, it _is_ possible that a pet lens _or_(not “and”) mounting your PIR four feet off the floor and upside down will work. Sometimes it does---but don’t be astonished if it doesn’t.