The reason I am apprehensive to migrate my wife’s content to my machine is that she has different apps. Some apps (and their fonts), plug-ins, etc., are not 10.6 compatible as she is still on 10.4, and some apps are the PPC specific versions.
Ergo, I fear that migrating her apps to my machine would cause devilish problems I could not find & resolve.
A migration can focus on just the user account, so —unless these apps you're worried about are in her home folder —then they won't enter the picture at all.
Will MA me to allow migrate only her Mail?
Nope.
What is wrong with the method I originally suggested, e.g., just moving her ~/Library/Mail folder from old machine to her new iMac?
Perhaps that will work. If her account already exists on the new machine, then that may even be mandatory at this point (i'm unsure as to how Migration Assistant interacts with existing accounts).
For sure the best way is to use MA to copy/create the account in the first place. [plus doing a migration that way also provides a full backup of that account's docs, bookmarks, events, notes, music, etc., ... not just email.]
