Your breastmilk is the best possible thing for J while you're on ABs as it will help protect her gut with all its goodies in it. I second the barrier cream suggestion for the horrible poo, if she gets a dotty rash on her bum you might like to use some canesten on her bum under the barrier cream as well. Keep an eye on her mouth but hopefully you won't have any problems.

Also now is not the time to be wearing plastic-backed breastpads - use clothies if you can and make sure the girls get an airing. This alone can help with the condition of your nipples a lot.

I was on heavy heavy duty ABs for a long time after my DD2 was born (CS wound infection plus various bouts of mastitis, etc) and she never developed thrush, only the bad runny poos.