My two Freyas wear 16mm eyes, so I can't be of much help with that without any 18mm to test.
In term of Barbie pants, the pants legs would be long as they are closer to 23-25cm dolls in height. Mind her feet if the pants are the tight type.
As for Barbie tops, I don't really have Barbie tops, mine are mostly Azone, Jenny,and Rement. I have an Irish Barbie dress but her paws won't go through the sleeves
So far Azone( S-Neemo tough back closing, L-Neemo/Obi27 top in above pic is nice), Jenny, Moxie stretchty pants and Rement stuff that are sleeveless should be okay. So unless you're prepared to unstring her paws to change clothes, avoid the non-stretchy long sleeves
More L-Neemo gear with Kitty! Taki and Kitty! Helen