Yes. As long as your output dragforce is less or equal to the weakest link in your setup you are fine. In this case your line of 5.5kg. You could even use a reel with the max drag of 21kg if you would like, as long as your drag doesnt go above "5.5kg" (in this case).
A reel with max 21kg that you can adjust in 12 steps (as an example only) would mean it applies (21kg/12steps = 1.75kg) 1.75kg per step, so 3:rd step in the drag would be 1.75kgx3 = 5.25kg of drag, and would also be fine with the 5.5kg line. Would apply for instance 5.51kg of drag, then its gonna break.
And if you were to have your 6kg rod with a 7kg line on it, then your rod would be the "weakest link", and the same math applies to that, keep the output drag less or equal to "6kg".