Lol about Tifa not knowing Cloud! Tifa is described as being the woman that understands Cloud the most. xD Just because she doesn't know EVERYTHING about him, doesn't mean that she doesn't understand him, or whatever. You can love someone without knowing every single detail. I think it's the RF states that even Tifa doesn't know all of the complexities of Cloud's heart. I think it's interesting that they word it as even Tifa meaning that Tifa knows the most about Cloud. Understandable, considering they shared their feelings with one another, and especially after the Lifestream scene. It's because she knew the most about him in the first place, that she was able to help put his mind back together again. =/
I would like to see one instance where Aerith is shown as understanding Cloud more than Tifa. Keep in mind that Cloud stated it's Tifa's opinion that matters most to him, haha.