What i'm asking is how are Hal and Guy able to fight the Guardians of the Universe, while the guardians are fused with the entities? The reason i'm asking is because shouldn't a guardian infused with an entity be far too powerful for a single lantern too handle? Not to mention powerful enough to crush an entire universe?

I would think that the presence of the entities might disrupt their own powers. I would think some of the entities would be fighting each other, or some would at least want to help Earth's Green Lanterns and fight against Krona's control.

Geoff Johns is writing it. It doesn't have to make sense. Hal Jordan can do anything.

We haven't even witnessed the battle yet. Victory over Krona will probably depend on John and Kyle freeing (disabling?) Mogo.

yea so far there hasnt been any fighting. Also while the guardians are powerful they are emotional closed off and neutral so i dont think that would help them very much, it would be like the compassion entity taking over Darkseid or Doomsday. They are emotionally closed off to anything but rage so it would be hard to use.

a better example would be Hal, John, Kyle, and Guy all having to figure out and get used to these new rings bc they are used to using will alone. So after eons of not having emotions, the guardians need to figure out how to tap in to them again

Is there a white lantern guardian? 

no