Only issue I have heard is about lack of headroom during fermentation. You can reduce your recipe or split it into two of them , but that leaves a lot of room for O2 to cause issues too. The 6 and 6.5's are designed to allow enough headroom so blow offs are less likely to make a mess in your ferment room. A good plan since it is likely would be to install a blow off tube early and switch to an air lock after the active part of fermentation has subsided a bit. I am still learning, but have read a ton on this since I have access to a lot of theose 5 gallon bottles too.