Demegol helmet without SIGNIOFICANT modifications should not be used on a Modern Era Mando kit at all as it is a KOTOR Era helmet.
Legacy Era is classed as Modern Era for the time being, Legacy style helmets and Modern Era canon and custom style helmets are appropriate.
this is just a personal thing of mine, has NOTHING to do with approval but, if you're goign to do Legacy style plates, do a Legacy style helmet. To me, mixing them doesnt look right. The design, while perhaps not everyone's favorite, was artistically designed to go with the armor. The Fett style (and it's custom variants) was designed to go with a whole different style of armor. To me, it looks really wierd when mixed. Kinda like a ManBearPig.