If I have a skill say fireball as a 6L in my chest and have combustion linked for the negative fire res,
would that combustion stack with let’s say wave of conviction cwdt with combustion linked to that.

Because they are separate spells would it further decrease or just refresh it?

no it wont stack

Combustion will apply the fire debuff, while woc applies fire exposure

But the separate entities of combustion will not both apply unless the levels are sifferent