Always put the corrosive on the huge if you have one. This way you completely negate its only downside, the lack of ammo. By the time all your other multicannons are empty, the huge will still be firing, applying that corrosive and doing good damage. Additionally the low fire rate means that down time for reloads is also less (relatively) compared to other sizes, making autoloader less effective. If you put corrosive on a smaller hardpoint, it will run out before the huge, and the huge will not get the 25% benefit for some of the fight.I've had good results with autoloader. So long as you don't use rapid fire, you'll never have to worry about reloading. Incendiary has it's uses if you're running a full multicannon build and you need better shield stripping. The rest will either only see a use in PvP, or aren't worth it.
Most of my early builds were efficient beams, overcharged multis with one corrosive and any others autoloader. If I'm guessing correctly and you're using a huge and two large multicannons, put the corrosive effect on one of the large ones so autoloader can go on the huge and the other large. You'd want to keep that big one firing.