Moreover, can septum ring be too small?
You still don’t want the diameter of the ring to be too tight. If it is too small, It can actually cause migration. Because the ring is smaller then the space between the piercing and the ends of your ear/nose/lip, that extra tissue is going to end up compressed.
Beside this, what is the smallest septum piercing size?
Generally 14g / 16g. 18g is the smallest size that you can use once your septum is healed, I heard that anything smaller than 18g can cause the “cheese cutter” effect.