Can I breastfeed using miconazole?

Yes, safely. Miconazole does not enter the blood stream from the vagina to any significant extent and is safe to use during breast feeding. If you are thinking of using it on your nipples, then Clotrimazole (cream) is the same medication used to treat oral thrush in a baby, would be safe to use, and be effective as well.
Yes. I assume you're using this topically? Miconazole can be used to treat fungus in infants as well, so should be safe to use with breastfeeding. If applying to sore nipples, wash them off before feeding.