Rats fighting...one is now injured

Question
I recently got two male rats...they are little boys, less than a year old.  They got along great at first...I got them from a breeder, both in the same litter, so they grew up together.  However, they've recently started fighting.  I read up about it because I was worried and it seemed like it was a normal thing because they are just two little boy rats going through puberty...and they do fight.  Also, my gray little rat (I have a black one and a gray one) was getting a little chubby and I noticed he started hording all the food that I put in the bowl, while the black one remained pretty skinny.  It became pretty clear that the gray one was the "dominant" one.  When they fight they usually rustle around a bit and then calm down and they calm down immediately when I get to the cage, but tonight it was louder than usual...I heard a couple squeaks and a lot of rustling (this is all from what I heard upstairs with the rats downstairs) so I rushed down to see what was the matter.  The gray one was out and about and the black one had retreated to a little tunnel I have in their cage...normally they both come right up to the door when I come up to the cage, but it took some coaxing to get the black one out.  I gave the black one a treat and while it was eating it in my lap I noticed a big gash right behind it's ear...I have the two rats separated for the time being, because I don't want the black rat to get hurt more...but I'm not sure what to do.  Do you think it would be serious enough to take it to the vet?  Also, should I just keep the two separated?  Or is the fighting something they will get over in time?  Lastly, about the food hording thing...is there something I can do to get the gray rat to stop doing that?
Your help is much appreciated...I'm very worried about my babies...especially the poor black one right now.  I would hate for anything serious to happen to them.

Answer
You need to let the breeder know there is aggression in the litter and consider having them neutered at the breeders cost.

As for hoarding food, this is commonly seen among rats that are fed on a schedule but sometimes they do hoard regardless.  Do the rats have food available at all times?

Is your vet a good exotic vet (board certified)?