Hotheaded Red and all -- Judaism puts the health of the individual before all laws -- for instance, if it is a day of fast, but this would be injurious to a person's health, that person is enjoined from fasting; a sick person may travel on the Sabbath to a hospital, if not doing so could jeopardize his life; and so, too,  a circumcision can be deferred until a baby is strong enough
to survive it.  Interpretation of what is injurious can vary widely of course --
I know that premies circs can be delayed, religiously, until weight is gained.

As to breastfeeding being established, I would expect it to be a matter of individual cases -- it could be argued either way.  On the one hand, since the baby is not breastfeeding yet, an additional interference by circumcision shouldn't make that much of a difference.  On the other hand, if the baby is just now starting to latch, maybe we shouldn't interfere, because the Torah does prescribe two years of breastfeeding, and this would enable it.  On the other hand, there have been always been substitutes for breastfeeding, and children have survived.  On the other hand, even one bottle is not a good thing, 
so we should do everything we can to ensure the least number of bottles, etc., etc., etc.

(This balancing act is called "pilpul," and the final decision would depend on how orthodox the sect is, and what their particular views are)

Chanita