Hai sayang, first of all you're doing so well as a first-time mom, mixed feeding is absolutely fine and many mums here do it too! One thing worth knowing is that breast milk digests faster than formula, so your baby may want breast milk feeds more frequently, while formula keeps them fuller a little longer. Someone in my mum group mentioned it helps to alternate the feeds separately rather than mixing both into the same bottle, so baby's tummy can handle each one more comfortably, a few mums noticed less fussiness that way. Some mums did share that switching between the two caused some mild digestion changes at first, so just go slowly and watch how your little one responds. That said, do check with your paediatrician too, as they can give the best advice based on your baby's specific needs.