Ugg, sorry you're dealing with this. GBS is a pain.
I was GBS + with my DD, and had decided to skip antibiotics and do all I could to improve gut flora (probiotics, vit. C, Goldenseal after 37 weeks, bee propolis, zinc, etc.- I can't remember what else exactly I was supposed to take but it was basically all stuff to boost the immune system.) I had also planned to follow a hibiclens protocol during labour, as according to my research at the time, that method has been used in Europe with essentially the same success rates at antibiotics. I was however open to antibiotics in the event that any of the factors that increase the risks of GBS infection were present (PROM, premature labour, maternal temperature increase, or other signs of infection.)
As it turned out we did have early rupture of membranes, and I ended up with fairly massive quantities of antibiotics (I think about 8 IV doses over 30ish hours.) I'd decided if we did need antibiotics, to make sure I had probiotics for both me and baby right away and it seemed to help as we had no issues with thrush, or other tummy trouble. We did have some nursing issues in the early days, but nothing related to the antibiotics. A friend who ended up with lots of digestive issues and a colicy baby following antibiotics in labour tried newborn probiotics after a couple weeks too, and found an almost immediate improvement.
This time if I end up GBS + I think I'll try the same herbal protocol and retest if I have time. If not, I'll probably just go with the antibiotics, and plan to use probiotics after as well.
With a penicilliln allergy, I think I'd be tempted to bring up other treatment options with your OB, particularly the use of hibiclens rinses during labour and see if he has any opinions.
Good luck! I found this to be a really tough decision, as there are strong positives and negatives on both sides of the issue.