You will have increased suitability for sure, but you will use a lot of helpful passives from the Medium armor buffs; especially for a Night-blade.

BUT.... that all depends on what you are going for... so...... give it a try.

Armor differences are as follows:

Light armor has 25% armor rating (example, a light armor helmet might have 250 armor value)
Medium armor has 75% armor rating (example, medium armor helmet with 750 armor value)
Heavy armor has 100% armor rating (example, heavy armor helmet with 1000 armor value)

Other than those ratings and visual differences, armor types are all the same before you get into skill trees. The skill trees are really what make the armor types stand out from each other.

The Light Armor Skill Tree provides you with bonus magicka regeneration, reduced magicka costs, increased spell damage, increased spell penetration, and increased spell critical chance. The Medium Armor Skill Tree boosts your stamina regeneration, reduces stamina costs, increases weapon damage, increases weapon critical chance, reduces the costs for sprinting and sneaking, and reduces the detection radius when sneaking. The Heavy Armor skill tree increases health, health regeneration, armor, spell resistance, reduces the cost of break free, and returns some magicka/stamina to you whenever you get hit by an attack.

Because of those skill tree bonuses, you always want to focus on the armor type that fits the specific role you're trying to fill:
Healers and Spellcasters want light armor to maximize the effectiveness and efficiency of their spells.
Fighters and Archers want medium armor to maximize their effectiveness and damage with melee/bow attacks.
Tanks want heavy armor to maximize survivability/damage resistance and restore some resources while taking hits.

In most cases, it's always best to go 7/7 for whichever armor type fits your chosen role. However, it's acceptable to go 2 heavy armor with either 5 light or 5 medium to boost your damage mitigation as a caster/melee damage dealer. Once you unlock the passives in the undaunted skill line, it's often preferred to go 5 of one type of armor, and 1 of each for the other types, to maximize the passive magicka/stamina/health bonus you get from the undaunted skill tree (example: 5 medium, 1 heavy, 1 light).

If you're wanting to play a battlemage/spellsword type character, who uses both melee weapons as well as heavy spellcasting, you'll need to mix medium and light armor in the ratio you prefer. This is not usually recommended though, because you need at least 5 pieces of one type of armor to get one of the passive bonuses in the medium or light armor skill trees (bonus spell critical or bonus weapon damage), and those benefits are huge. Usually it's best to go 5 light/2 medium and focus your attributes more towards stamina, or to go 2 light/5 medium and focus your attributes more towards magicka, to make up for the difference.

Hi,

What i always do when levelling is the following,
for my stamina NB i go 5 Med, 1 Heavy(Training Trait) and 1 Lite(Training Trait)

All the armours have a skill and passives which could be beneficial at some point so i try to level all my armours to 50.

Then you are not limited to only the armour you have levelled.

on my VR14 NB i run only MED armour with Nirn Trait on 4 pieces,
but as a tank i would probably run 2 Heavy,

The benefits from Med armour are mostly related to stamina(regen and reduction)