I supplemented with dd and just started this week with DS when I went back to work. I found that it was much better tomgive mixed bottles of Ebm and formula instead of 3 Ebm bottles and one all formula. You will probably find that the taste will work better for your LO.
Also, gerber amd similac both now make a supplementation formula that is specifically for breastfed babies who are just getting some formula. He just got his first 3 oz of that today, but my ped recommended it, and I never knew it existed!
We have also had am increase in gas since introducing formula even though he is only getting like 3 oz a day. My dd didn't have this issue, but the nurse I spoke with said that he probably just needs to adjust to it, so keep that in mind. Also, even with only some formula you will probably smell a difference in your lo's gas and poops (and it's not good!)