Is it bad for baby to use different bottles with different nipples?

My mother keeps insisting that using two different bottles is confusing for my baby and that it's bad for him. She keeps begging me to pick one type of nipple and stick with it. The only reason why we have different type of bottles is because the mendala ones are compatible with my pump and my mom insisted that for his gas we got advent bottles. I've noticed zero different in gas with using either of the bottles. He struggled with using the advent bottles at first but now is used to them. We switch back and forth depending on which bottles and nipples are clean and I don't really see the issue with it. My mother is making a bigger deal out of it than it is, I don't really see the problem with switching back and forth. What are your thoughts?

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