Yes, you didn't do anything wrong to make them act that way. Petsmart did you a disservice by not warning you of this fact. Vanillarat is right, while some young males who grew up together can live their entire lives sharing a cage and not fighting, most often they can't coexist. You will probably need to set up separate lodgings for the boys. It's sad for us to watch them live solitary lives, but unless you want fighting by mixing boys, or want to be hip deep in babies by mixing them in with the girls, they have to be. However, if you play with them daily, they will still be very happy little guys. Once they're tamed, they make the best pocket riders.
If you do want a colony, then get females only.
With one notable exception where one very terretorial female mouse refused to allow me to introduce any new mice to my colony without a bloody fight, my female mice lived together well their entire lives.
In that story, I'd been gradually obtaining new mice to add to my collection for several months until I got a young blaze mouse named Crash and a capped dalmation named Boomy. Once Crash and Boomy were full grown, I'd wanted to add a couple more. I bought a little silver youngster and the only thing I saw the entire first day was the usual minor squabbling to determine pecking order.
I checked on them in the morning and my little silver mouse, Mrs. White, was savaged bloody all over her hind end and perched at the top of a toy ladder, looking stressed and shocky. I seperated her into a small 'hospital' container and treated her for the next week. She pulled through, though she lost all the hair on her bum. I waited until her hair grew back and she was bigger. Then I carefully reentroduced her to the other girls. Again, the only real problem was Crash and Boomy. But, I think Mrs. White had had enough and she went Bruce Lee on them up one end of the cage and down the other. They both retreated to the far corner and Mrs. White had no problems integrating into the colony after that. I always called her "Mrs. White-Pale-and-Tragic" though I think all three of those girls were aptly named.
They all got along after that, but I didn't make any attempt to introduce new mice to the colony after that. I'm sure it could have been done, but I just didn't have it in me to risk another mouse that way.
Best of luck! Post pictures! We'd love to see them