I would not mix formulas for several reasons. 1 baby tummy can be sensitive to different formula. Can make baby cranky. 2 it came in the mail. Scary;/. Also mail is transported in all temps so formula can be bad with the nutrition depleted. I know formula is expensive but to me it's not worth taking chance and Not worth a fussy baby.
you can try it, but with my daughter switching made her stomach upset and spit up became worse. I wouldn't recommend it but if there are medical reason, like she isn't keeping the other down then I would try it. doctors usually suggest you don't mix.
Yes, it is ok as long as they are of the same kind of formula though of different brands. This is because same types of formula often have similar basic ingredients. However, I suggest you get more information from a pediatrician.
First ,GET OK from the pediatrician, because your baby may have reasons it may not be a good idea . All formula companies send free 100.%sealed , safe samples In the mail,and the hospital and doctors office as well. ALL formula is FDA approved. You find the one that suites the baby best. So unless the baby has a particular issue( gas, etc).use the Free samples! ( baby may like it more , you will find out right away. some people won't mix, but it's fine)