The sooner you go back to your desired routine the better. Babies are generally quick to adjust, especially when they are so little. If you wish to combi feed, do it and stick with it.
They should be fine. Their poo might change a bit but that's because breast milk goes through them quicker
I did this and had a random lapse of routine with it. My milk took forever to get in, and just lost in the whole newness of motherhood. We didn’t have any problems, tummy wise, with it! But of course, every baby belly is different l