Sounds like a gorgeous tank!
I've had pandas in my 20g for a few months now, and I notice a definite improvement in the look of the sand. They're very industrious; if they're awake, they're sifting through the substrate. I still need to vacuum, but it's just fish poo I worry about now, as the cories take care of all the uneaten food. As far as I remember, they're not a heavy bioload, so a school of six (more is better-- I'm currently trying to up my school to nine or ten) shouldn't be a problem, especially if your tank is heavily planted. They've also never bothered my livebearer fry. They're omnivores, but their mouths are teeny tiny and they're not aggressive in the slightest.
My only concern would be the gravel. Cories have sensitive barbels, and anything rough or gritty could damage them. It's hard to tell from the picture on Caribsea's site, but is your substrate rough and chunky, or does it feel rounded and smooth? They're also very sensitive to changes in water parameters and easily stressed, so you'd really need to drip acclimate them, or you're likely to lose half of the fish you add. I know from unhappy experience; I took a couple hours acclimating my first bunch, and it still wasn't slow enough. I'd keep the tank lights off for the first day or so, too.
They are adorable once they get settled, though. I originally got mine to serve as a clean-up crew, and now they've ended up stealing the show from my other fish.