Wow, that's an interesting challenge!
I've never filmed from a balloon but I've taken photos from one, and the main thing I'd observe is that while in the balloon you typically have no sense of movement, even in a strong breeze, unless you look down and see the landscape moving under you. It's a very stable environment in the basket. A good pilot should be able to keep your balloon at a safe but close-enough distance (and consistent altitude) for you to get your video, but you'd have to consult with pilots to find out what that distance is, which in turn will help you figure out how much reach you need from your lens. I don't think they can fly very close to each other; the risks are too high.
I don't know if you're recording any sound as well, but be aware that the gas jets used to fill the balloon with hot air (and used periodically in flight to maintain or increase altitude) are really loud.
My experience in a balloon was a long time ago (late 1980s) when I volunteered for a week at the Albuquerque Balloon Fiesta. They probably have guidelines for safe distances, since so many balloons are flying at once.