Switching formats mid-roll is less interesting than you might expect. You lose film between formats, so I'd only do it if it was a significant change (say between 6x6 and 6x12), and I only needed to do it once per roll. Maybe twice for 220, if you happen to have 220 around.

The Sinar Zoom is remarkable for what it can do, and the film flatness is excellent, but there's a lot to go wrong, and you need to handle it carefully. No problems with mine, but I've kept a Linhof 6x7 back as a backup.

The Chinese backs usually come with masks that you can change between rolls, which is usually sufficient, and there isn't much to break on them. The only potential worry, I think, is light seals, which could be a problem with many backs.