I was misinformed. I do not have all of the answers!
It also makes sense to start leveling units like Narcian with the lower rarity version - you can farm +sp from killing against low level enemy 3 times that way. At 2*...3* and 4* ( well and 5* if you can upgrade him that much) ...so this helps out learning skill more efficiently, for units that will have trouble to get the kills on higher difficulty.