we use mostly medela bottles because I have the medela pump. Have also used avent and playtex vent aire.
the small 5oz medela bottles and avent bottles come with slow flow nipples
the larger 8oz medela and avent bottles come with the medium flow nipples
the playtex bottles were 6oz and came with slow flow as well.
in my opinion, you don't need to go out and purchase different flow nipples because its not like your nipples open up more or change flow rate...and the faster flow just becomes more dangerous with choking.
some babies prefer one type/brand nipple over others and so i wouldn't stock up until you know for sure. we had these three types at home and interchanged them and our LO never had a problem with any of them, but some babies only like certain ones.