Right, I was about to say it seemed like that was being overlooked rather.
So if you have say.. a level 50 Miqo'te, and start a Hyur, that level 1 Hyur won't be getting a bonus on account of you're having a level 50 Miqo'te.
What will happen however, is that Miqo'te would get an experience bonus should it subsequently take up a Discipline it hasn't done anything in yet - a bonus that will shrink as the difference between that Discipline's level, and its level 50 discipline, shrinks.