Car seat inserts

I know you're not supposed to put anything in the car seat that doesn't come with it but what if my baby is too small to fit it correctly? My first baby was barely 5 lbs but that was almost 10 years ago and safety rules were different back then. He had a preemie insert and was fine. My daughter who is now 6, was almost 9 lbs at birth so I didn't need one with her. I'm not sure what to expect with baby #3 as my other 2 were polar opposites and they have a different father than this one.