Tell me anything, and everything!

I think my one concern is does it mess up their stomachs/digestive system?

I'm just barely meeting DD's demands... She has a huge bottle preference and I just can't pump enough esp at night for her. And WOH too so I don't really have time to power pump, add another pump session, etc... So I'm doing some prep work in considering supplementing a bottle of formula a day (I'd still pump for the miss feeding of course).