Migrating with different operating systems ... Issue? Trying to migrate software on two OS X macs with different operating systems. Source is running mountain lion and target is running Yosemite. Is this an issue??  I'm using an ethernet cable, have turned off firewall and wifi.
No - Migration Assistant always supports taking an older system and/or backup and migrating those Apps, Documents and Settings to a newer OS.

You generally can't go the reverse direction so Yosemite to Mountain Lion wouldn't be a possible migration path.