The motion sensors should be pointed away from windows and doors to prevent false alarms,
If your cat is a normal size cat, and not a lion or something like that, you should be fine. Top of my head, the motion sensors are good for 75lb. However, it depends on where the motion sensor is placed. If the motion sensor is on top of a kitchen cabinet, and the cat can jump up there, it will detect it.
To make sure, place your system in test mode, place the catfood in view of the motion sensor, arm the system and see what happens 
I think you'll be fine!