Ninja should be categorized as ranged physical.

[Jandor]

The easiest way to achieve balance, assuming that's the ultimate goal, would be to add a Scouting melee job.

10 melee. 4 tanks, 2 maiming, 2 striking, 2 scouting.
10 ranged. 4 healers, 3 physical ranged, 3 casting.

Moving Ninja to ranged doesn't make much sense, because it's not a ranged job.