Formula and breastmilk - compared

Can we give formula and breastmilk on same feeds or alternatively?.Like breastmilk at 6.am and formula feed at 9am?. Are there any disadvantages in doingso apart from gas and poop changes ? I Am pumping but not making enough supply ...my supply is only till his next feed., so always have formula ready if the pump output is low. I Am 11 days pp and have a medela pump hands free