My daughter was chronically constipated when she was an infant through about 6 years of age. She would have bowel movements that would have hurt an adult. We tried everything!!! Citrucel, prune juice, apple juice (that is constipating), white grape juice, cherry juice. Nothing worked (except suppositories) until we got her on Miralax at age 4.
My 2 month old neice has Hirschsprungs disease, where it is actually painful for her to pass gas and have a bowel movement. She is given either suppositories or PEAR juice. Apple juice may have alot of fiber, but it is actually constipating. Prune juice causes too much gas for their little tummies to handle. When she has Pear juice, she goes easier and more frequently so it is not painful for her.
Mixing the 2 isn't a good idea, you said she had trouble digesting the milk based formula, which I assume gave her diaharreah, correct? Mixing the two is just going to confuse her digestive system as they need to be broken down differently. I would try some pear juice and if that doesn't work talk to your doctor about Miralax, my daughter was on it from 4-6 and she has no problems now. I know it is a laxative, but it is one that actually trains your bowels to work correctly so you do not need to be on it forever and you don't become dependent.
Be careful with Juicy Juice brands as in all juices except white grape have the main ingredient as apple juice so you will not get the full effect. Gerber makes pear juice, it's found in the baby department at every store we have purchased it from.
Good Luck!! I hope some of this helps.