Taking probiotics once I found out I was GBS positive was what I did last time. I usually don't have any yeast issues but I am sure it was helpful in maintaining my good bacteria and was thankful I didn't have any thrush/issues with breastfeeding (just the minor food allergies which who really knows what caused that!).
It helps to take a few different high quality formulas I think because they all have different concentrations and types of probiotics in them. Right now I am taking jarrow, new chapter and fem dolphilus on a rotating basis. Also, I would suggest buying an infant formula to give to your baby when he/she is born as well. I am planning to do that this time and wish I had last time! Probiotics, in my opinion, can only be helpful and never hurtful!
I have heard great things about http://www.rockwellnutrition.com/HLC-Neonate-Powder-by-Pharmax-40Non-Returnable41_p_60.html (from the mothering allergy community actually) and moss nutrition Klaire Labs Ther biotic makes one for infants too. I have the Klaire labs one from my son so I am going to be using that one.
I looked through the cdc information and am resigned to the fact that if I don't get antibiotics in time the ped will make me stay for at least 24 hours and I am hoping I can convince her to let me go after that (if my labor and birth go quickly and my water is not broken for a long time) so that means at least one night in the hospital. At least I can prepare me and my 3 year old for it (never spent the night away from him before).