Is it okay to add formula to milk for a toddler?

Hi! My daughter is now two...around 18 months of age, she was taking horrendously long naps and was in a very low percentile for weight. She had always been an exceptional and gluttonous eater but suddenly got very picky around 14 months. She was switched to whole cow's milk at about a year so that wasn't an issue. Around 18 months, I started to add formula powder into her large straw cup of milk (without water) to give her more calories/protein/nutrition. Immediately, her naps shortened to a more normal time frame and her weight and height began to increase. She's doing great, according to her pediatrician, but it crossed my mind today that perhaps the formula/milk-sans-water combo might be too much on her kidneys? She doesn't eat a lot of protein ...it seems she's more apt to eat fruits, grains and veggies so I don't know if the milk/formula combo is ok or still too much. She gets her milk/formula with meals only and I give her water or watered down juice throughout the day. Anyone have any idea if there's a health risk with this?