Re: baby in 5th percentile at 2months
First of all, someone has to be the 5th percentile! So that in and of itself it not a problem.
So I'm understanding that she gets bottles of breastmilk from the sitter and DH? If that is the case, I can't see any reason you would ever need to supplement unless you can't pump enough. Formula would be suggested by someone who thought the baby wasn't getting enough milk at the breast for some reason. If she were having a problem at the breast, as soon as she got a bottle she'd be gulping down a ton.
How long is 'sleeping through the night'? Maybe sneak a feed in when you go to bed if you want to get a little more into her.
My dd1 has never been above the 10th percentile. She's 8 now and can easily pass for 6. I'm short and my MIL's family is tiny too.