Theres not many Salmon you wont land on 2kg, theres not many Snapper you wont land on 3 kg.But then again I wouldn't fish any lighter for Whiting than 3 kg. But it all depends on your angling ability and how good your gear and terminal is. (not how expensive). Forget about line class so much ..finding the fish is the key to fishing.

Are you intending to fish for points for a club or tournament?  Or are you just interested in having a bit of fun?  How much fun, time and patience do you have?

Line class fishing generally refers to fishing with line that won't break over the nominated strain, which mostly eliminates braid, and the fish has to weigh at least as much as the line class to count for points under game fishing rules.

For the salmon we've been getting around Perth lately, that weigh 5-6kg, a good angler should land probably >90% on 4kg line, in somewhere from 10-20 mins.  For 2kg, the % landed goes down and the fight time goes up.  Out in open ground around Cockburn Sound you'd probably land 75% or so on 2kg, but around Mewstone it might be less than 25%.  On 1kg maybe less than 10%...

Dolphin fish are pretty easy on 6, good fun on 4, a good challenge on less.

But it really depends on the species, and where they live.  Pelagic fish tend to be more forgiving because you are fighting them in open water, where there generally isn't structure for them to bust you off, and you are free to maneuvre the boat wherever you want.

Bottom and reef dwelling species are a different story, because the game there is often all about getting them away from structure quickly.

You could easily land a 30kg sailfish on 10kg line with 3-4kgs of drag in 15-20 minutes.  A 30kg GT won't be stopped by much less than 100lb braid with 20+ kgs of drag, but the fight is mostly over in the first few minutes.