mix feeding like daily bm and night fm? OR finish up frozen stash den take fm? will baby reject bm once she try fm? will she have probs like constipation or phlegm?

I am a working mum, so I keep a stash of formula at home for the nanny to use in case she runs out of ebm for baby. She usually ends up taking 30-40ml of fm every few days as my meetings get delayed and I am late to come home. I have run of frozen bm (only have 2 packets left) for emergency. She has no problems with either bm or fm though I have a preference to keep her as breastfed as possible for as Long as possible, but I try not to sweat the small stuff, so 40-50ml here and there is no big deal in my opinion. :)