I think that the best reason *not* to mix formula and breastmilk in a bottle
is that if you keep them separate and baby has had enough at that feeding from
the breastmilk bottle, he can refuse the formula.  If you mix the two, baby
gets formula in his tummy whether he needs the supplement or not.  I know
there are more science-based reasons than that (lactoferrin and all that) but
this explanation has worked for me more than once.

Lisa Jones, LLLL in Wellington, south Florida, USA