World of Warcraft & Overwatch – Can You Install Overwatch and Play WoW at the Same Time?

The World of Warcraft launcher will always patch the game up to the latest version and download any expansion content that your account is eligible for. It is very much dependant on the 'product level' of your WoW account. A lot of Cataclysm content was actually changes to 'classic'/'vanilla' WoW zones, this content will be downloaded regardless.

The WoW launcher allows you to 'stream' the game while playing, obviously this wouldn't be very good for doing PvP/dungeons/raids as you would essentially be downloading while playing, but it does mean that you can install just part of the game and still get into the game while the rest is downloading. It is also required to set the 'product level' of your account for your download.

Logging into WoW once the base amount of information has been donwloaded (ie: the launcher gets to the 'playable' stage) - will update the Launcher configuration file to specifically only download expansion information for the account you have. In your case it won't change anything as your account level is for all expansions but this can result in a much smaller download.

It's only a rumor. Though each vendor may give different pre-order bonuses for ordering from them (e.g., Amazon offers you a key to gain access to the MoP beta), Blizzard isn't actually offering any particular incentive themselves.