The Plasma weapon has quite an explosion. I think that if they were actually aiming for the Killie Dreadnought, then it would be totally caught up in the explosion. 20km is about right for the fleet that Casey took out, wouldn't you say? I mean, all the ships were much, much smaller, but they weren't exactly squeezed together either.
As for the Kraken vs. Plunkett, we don't know how good Nephilem long range sensors are. It's possible that a Plunkett with a good fighter escort could sneak up on the Kraken - once it's up close, the Plasma weapon is pretty useless, after all. However, this would depend largely on Nephilem sensor ranges.