Absolutely you can mix the 2. If powdered, mix the formula w water first, then add to milk. Don't mix w the milk to make liquid.
________
"Best" is whichever one does NOT cause:
- Vomiting
- Gas
- Diarrhea
- Constipation
AND
- stays fresh while they're drinking. (SUPER important for slow eaters, as many are sour in 10 minutes and rotten in 20)
That answer will be different for different babies.
For my son?
HAD HAD HAD to be "premixed", enfamil lipil (or a very exp specialty formula)
He couldn't tolerate powdered of ANY variety (including enfamil lipil)... And was a slow eater, so even though he tolerated a couple brands of premixed, they started turning sour or rotten by the time he was finished with them.
For his cousin? Similar Gentle Powdered ONLY.
His other cousin? Any brand as long as it was cold.
Seriously... The answer for 'best' changes with each baby.