BN plecos are typically more into a veggie type of diet.
I highly doubt that they would even attempt to or could catch & eat any of your fry.
As long as you have hiding places such as your rock I think you'd be just fine. I'm sure they wouldn't pay the fry any mind at all. I'd be very surprised if they did. I have Endlers in my BN tank & no problems with the fish or fry at all. Lots of adult BN, semi-adult BN & BN babies in the same said tank.
Just my HO.