I would suggest using the book Contraceptive Technology (I have the 16th ed.
1994) by Robert Hatcher MD (the nation's contraception guru) et.al. as a
resource for birth control during lactation.  It has an excellent section on
the progestin-only pill (perfect use failure rate is only 0.5% in first
year) and another section on Postpartum Contraception and Lactation.  In
that section, they say "Progestin-only contraceptives (POCs)...do not have
adverse effects on lactation, and some studies suggest that they even
increase milk volume.  POCs do not have adverse effects on child growth and
development.  Therefore, because their contraceptive efficacy is very high
and because they are simple to use, these methods ore excellent options for
lactating women...". (p.42)  The authors go on to say "Combined oral
contraceptives are not the contraceptive of choice for breastfeeding
mothers.  There is clear evidence of a reduction in milk supply due to the
estogen component in combined pills, even those with low-dose preparations."
(p. 443)

I have seen many women lose milk supplies on low-dose combined BCPs, and
have good success using progestin-only pills in my practice.

Gretchen, CNM