I've come on and found my mob farm blocked off and wondered why and found out... when the mobs hit the bottom, only one die and the rest of them that are stacked just stay there. I personally find this as a bug, and if you have a fix to it (Other than a crusher) please PM me. ~Spore
Auto killers are put in place to prevent lag, as it could have hundreds of entities colliding in a small area. The point of mob stacking is there's technically only one entity per mob group, therefore you can store hundreds of entities in a single entity, so no lag. Which means no auto kill is required.
I luv mob stacker, don't take away my bae! anyway yeah, I wouldn't spend time making an auto killer. If you already have one, leave it, but it's no use because all the mobs will be stacked into a couple entities (the reason I say a couple is because sometimes they don't stack properly).
The plugin is incredibly useful for eliminating entity lag from mob farms, and theoretically increases spawn rates of inefficient mob farms. It's very easy to auto kill then still, either by using a simple crusher or a lava blade. The only downfall is standard "fall-to-death" mob farms won't be an option.
Sorry if I'm moving this a bit off topic, but is there any reason why say, a stack of 5 won't stack with my stack of 300?
Providing they're exactly the same mob type, if they're moved close enough they should. If you have any proof of this happening, report it as a bug.