Official timelines (e.g. Cloud and Zack's 10th Anniversary Ultimania profiles) put Zack's death in September, though I'm not sure if it was clear whether it was toward the end or beginning of the month. But, yeah, it's unclear where in the space of time following that Cloud met Tifa. It had to be before December 9th, however, as that's when the game began.
As for Last Order, that's not canon anyway, so I wouldn't worry about it too much, guisado. It actually has a pretty huge continuity error, though, in that it gives the impression the whole series of events from Cloud and Zack's escape out of Nibelheim to Zack's death outside Midgar happened in the space of one night.
Tseng's opening narration says that they had escaped three hours earlier and the story carries us to the next morning just before Zack dies. Their escape actually happened in December of 0006, and Zack didn't die until September of 0007.
Another big error there is that Last Order places Tseng in a helicopter going to intercept Zack and Cloud while Crisis Core has him in his office at the time of Zack's death.
So, yeah, just ignore Last Order as far as continuity goes. Enjoy it for what it is (I still like to watch it sometimes), but don't take it too seriously.