That idea of Zack doesn't fit with what was originally presented in FF7 alone. Cloud definitely behaved like how he perceived Zack to behave at least. Even if you can argue that we didn't really get a reliable firsthand account of what Zack was like in the past, Aeris says that Cloud reminded her of him at first, so it seems pretty clear his surface behavior was similar to that of Zack. Crisis Core and all the other compilation stuff is retcon as far as I'm concerned.