Formula/pumping

I wanted to do both is that fine? Also I have so far only the Similac brand but my sister is telling me to also use enfamil because I’m guessing she wanted to buy me that brand (sorry if I spelt it wrong lol) but I thought it’ll upset baby by using 2 different brands of formula and she said she did it with her daughter and she was fine. I don’t wanna turn it down and be rude and have her waste money and not use it but I don’t really want to upset my babies stomach because she refuses to listen to me just wanting to stick with one brand