I also did combo feeding for a while before giving up at 7 mo. At 4 mo I came back to work, so the mid-morning feeding was from pumping (skipping that feeding allowed me to have enough to pump) or formula when i could not pump. And dream feed was always formula since 5 mo, as I did not have enough at 11 pm for him (he usually drained 200 mL at the dream feed). My supply was not affected at all. Oh, and even if I was bf, I went away for five days for work just before the 4 mo mark, I pumped for the five days, and left ds with frozen bm and formula. When I came back DS latched exactly as before, and I had no problem with the supply.