We did Karo with our younger 2. Not a lot, don't go crazy. Just a bit. It helped and it helped them not be constipated by the formula.
We did Allimentum, instead of nutrimigen. Our son could not process proteins from either dairy or soy and was on it until he was about 5 months old and his intestines had matured somewhat.
If you aren't using the ready mixed, try it. It is more expensive, but if we tried to give our son the powdered he wouldn't drink it. It actually smelled way, way worse. Like burned tires. You could smell it coming off the bottle. The ready mixed didn't smell nearly as bad. I was able to call the company and tell them that he wouldn't drink their formula and they actually sent me a refund for the two very expensive cans that I bought.