A Plasma weapon is effectively a Directed Energy weapon. Plasma does have mass, but the impact force of the Plasma isn't the most dangerous part about it. Plasma is hot. How hot? Electrons falling off their atoms because there's too much energy in the system hot.

Plasma Weapons aren't meant to kill like a bullet, by cutting through someone or blunt force trauma. They're made to cook you alive. The standard defenses against a bullet don't work here. Armor will absorb the heat, and a sizable chunk will still make it to you instead of being transferred into the atmosphere.

So any armor is going to need a cooling system.

Assume that your ranged weapon projectiles have a finite amount of energy to them. The cooling system on your standard armor is capable of taking [x] number of shots within [period of time] before heat begins getting through to the wearer. After [y] shots, the wearer starts to have a non-0 chance of permanent injury at best.

A bladed weapon is different from the ranged weapons in two ways. The first way is that there's more plasma to it. The heat transferred from getting the edge of the sword to contact the armor plating is going to be much higher than that from a shot landing. Assuming the temperature of the plasma around the sword and the plasma in the Plasma Pistol rounds are the same, you'll overload the Heat-Shielding faster by bringing more plasma into physical contact.

The second way is that there's still a cutting edge beneath the plasma. If the plasma super-heats the materials in the armor, that's going to make it a lot easier to cut into in most cases. Cut the armor, and you might manage to cut into the cooling system and cripple your enemy's defenses.

However, you're going to have to shield the sword's bearer against the heat from its own plasma. Again, these things are HOT. The convection off of those things will be deadly to be around if you're unshielded. I'd recommend that you have the plasma suspended in a magnetic field, which is itself within a field that produces a near-perfect vacuum to prevent heat bleed-off. That's what the lightsabers from Star Wars do, if I remember correctly.

The better solution would be for your Melee-Units to have better armor. They'll need that anyway, since they're going to be out in the open more often than not. If their armor can cool faster, they'll be able to take more hits before the heat starts getting through to them. They'll also not be cooked alive by their own weapons.