https://www.reddit.com/r/Stellaris/comments/4j3cns/minmaxers_guide_to_fleet_composition/
I think think 3S/5S core is the best. You want two loadouts:
1) 3 x your strongest non-missile weapon; this is your damage dealer
2) 2 x your strongest non-missile weapon, 1 x point defense; this is your anti-missile boat
3) As much shields as you can fit
4) Defensive AI
And then you adjust your Corvette doomstack's ratio depending on what ships the empires you're fighting against use. For my first time playing the corvette meta, which I used in an MP game, I went 4:1 damage/anti-missile and totally crushed another player's fleet. Their 2k power autobuild destroyers/corvettes vs. my 2.2k power corvette spam. I came out of that first engagement at ~1.8k power and them at 1k power (they used the emergency retreat). By the second engagement, I had wiped their fleet with minimal losses because my point defenses just stopped most of their damage.
If your opponents are not using missiles then just spam your damage boat.
There's some other strategies like where you spam Battleships with the healing aura effect (they stack, supposedly) but I haven't tried that yet. The only counter to PD for missile empires is swarm missiles, I think, but that's a mid-late game tech I think and I've never seen it.