I'd just keep doing what you're doing. If your horses were going to founder, I think that they would have done it by now. It's not like you're suddenly throwing them out onto lush grass that they've never been on before. Can you throw them into the paddock at night instead of putting them in their stalls if you don't want them in the barn?
Yeah - we can leave them in the paddock. It's just a little small for three, and they bicker and fuss, nip and kick at each other just for something to do. They get stircrazy.
~And~ we've had little luck trying to feed their grain (plain COB now) out there, Bay doesn't stay at his own pan, he steals from the girls, and they end up at his pan getting his pills. So we've been stalling them at least for meals anyhow.