I'd say the glamour quest belongs at around level 17. That puts changing your gear's appearance halfway between being able to change its color with dye and being able to change its stats with materia.

As for the level of the gear and glamour, I think it should follow the rule used in most other circumstances: Let people glamour anything they can wear. In many (probably most) cases, that would come out much like the current rule, but would be just slightly less restrictive. For example, a level 30 White Mage would be able to glamour her level 27 Velveteen Cowl to look like a level 30 Velveteen Gown since both are wearable but the only stat-useful one is a few levels lower.