I would have to agree with Vanilla's comments. I didn't read her links, but they're probably packed with valuable info. In my opinion, if you find a wild rat, it's best to let it go back into the wild. If it was born in captivity, then I certainly wouldn't let it go though because it might have a difficult time adjusting - not being able to survive.
Your adult wild rat, however, obviously knows how feed, and you never know if it has a disease unless you have it tested at a vet (those tests aren't 100% reliable though), and it could very well take on the "hamster trait" (feeding on the babies and smaller rats), so I wouldn't put it in the cage with the small rats either.
If it were me, I'd let it go back into the wild, for its own sake, yours and your current "captivity-born" rats.
If you do let it back into the wild, it'd probably be best if you let it go somewhere at least 10 miles from your home. I caught a wild rat once in my house (chewed up the cords to my stereo, desktop computer and laptop computer) - let it go 4 miles away from my house, and it came back! It didn't find it's way back after I recaptured it and let it go 10 miles away though. LOL!