I mixed-fed from pretty early on with ds3, having had lots of problems feeding ds1 and ds2 - I did my best to ebf ds2, but he ended up in hospital with them saying he was 'failing to thrive' - and he only started to gain weight when I introduced formula feeds.
With ds3, what I did was to breast-feed during the day, and then gave formula for the last couple of feeds of the day - just before my bedtime and the night time feed.
I found that, if he had no formula during the day, he lost weight, if he had just one formula feed a day, his weight stayed the same, and with two a day, he gained weight sufficiently well to satisfy the HVs.
Of course, you might find that it works just fine this time, Flossy - each baby is different and you might find you have none of the feeding problems that led to dc1 becoming dehydrated - and this time you will know much better what you are looking for, as signs that there may be problems developing.
How would you feel about giving breastfeeding a try for the first few days, and monitoring closely to see if your new baby is doing OK on that, and getting enough hydration? If he or she does no wet nappies in 6-8 hours, that is a sign of dehydration, or if the soft spot is sunken, if the baby is lethargic or if their skin is dry and cool, and stays up if you pinch it gently up between your fingers - and you could immediately introduce a formula feed.
Whatever you decide to do, I wish you every happiness with your new baby and their older brother. 