Mixing Formula Brands - Can I do it?

Ladies,

I need your help. I had a baby boy recently and I'm giving both breastmilk and formula to him for various reasons. I'm currently using Enfamil newborn powder formula. However, I have got few Gerber powder formulas to use. I read somewhere that we need to use the same formula brand for the baby and shouldn't mix brands. Is that the case? I want to know if I can use those Gerber formulas and I can mix up formula brands in the future. Can you help?

Also, if your baby does drink the formula that you mixed, how long do you keep it for next use?

Thanks!