And for the free ranging I have only done it outside in summer and they will follow you around in the grass and will never get too far from your feet Like ducklings LOL And, although this is not proven, My four rats all lived until they were three to four years old and I never missed an occasion to take them outside to run and get some fresh air. Compared to my friend, who's rats died after only a year and a half and she never let them get too far from their cages. I personnaly think that it is a great idea to let them run around outside! Also if you do have only a small balcony what I did was I put a box of dirt with a few treats and seed hidden inside and they had so much fun with it!! sure they get messy and you have to give them a bath, but honnestly it is worth it!
90% chance they won't, domestic pets like rats are dead scared of being left alone in an environment they don't know and with rats, I've found they always run to you when they get startled or nervous, not into hiding. Someone left a rat outside "for science" to see if it'd cope, and the thing sat there frozen for the whole night - in the morning, this sad excuse of a person found some fur and blood on the spot it had been. It didn't move even when a predator caught it.
So, I think you can freely let them free range. Take them out with a "safe spot", like a pet carrier with their own stuff like a blanket they've used inside, sit near them and make a ring with your legs where they can explore at first. Then you can turn them back with your hands if they seem too eager to jump out of that "ring" you've created, and most likely they will only step outside the box for a moment before cowering back in. They never charge out to the unknown, they go for the safe spot they know.
... and keep an eye for predators. I'm paranoid about a hawk randomly appearing on my face when I go out with my rats - even on my own balcony, lol. I haven't heard of a predator striking on a pet rat in broad daylight on someone's yard EVER, but knowing that doesn't make me less of a nervous wreck. xD
Bill me if they disappear - watch for potential hideouts though and avoid them, just in case! Someone had lost their rat to a sewer opening, the rat fell in and never came back. This wasn't on a safe yard though, it was in the middle of the street where the rat probably felt threatened by all the noise, people and smells around her, escaping to the only available quiet and dark place when she fell of her owner's jacket. Personally I'm not brave enough to take my vermin out when I go to shop or anything, mostly because of dogs and such, but with very bad luck a... dynamite could explode in the coal mine below your yard nobody knew of, and your rats could charge for safety. Choose a place where you and their box are the two safest spots on Earth for them.
Overall, there isn't much that can go wrong if you're cautious and keep an eye on your boys. Rat loyalty and trust is an amazing thing. xD Good luck!