Really depends on how how you are fishing for them mate.
Are you going to livebait for them or spin...??
With kings a large line capacity isnt overly important as they fight very dirty and will hug the reef in close.....and try to bust you at every possible chance.
When I livebaited the Sydney ledges I fished 100lb mono straight through and got busted numerous times by BIG kings.
If you want to spin for Mackeral and kings and want just the one outfit to cover both....Get yourself at least a 10ft rod rated up to 10kg and something like a Shimano TSS4 loaded with 30lb (braid) or if your prefer mono go 20lb for a bit more line capacity.
There are other options but this is a relatively cheap set-up.
Its a bit of a general question you ask though...??
For Spottie and school mackeral you can use 6kg and have some great fun....
You can also stop small kings on the 6kg outfit....Once over 4kg though they can be hard to stop off the rocks on 6kg line.
If you want spaniards I would be using the heavier set-up as the line capacity will be more important with them....as it will be for big tuna.
Have I confused you...???
Pete