An 8-speed chain will work with no problems on a 7-speed drivetrain. Still, I'd suggest investing in a chain wear measuring tool so you'll know exactly when your chain needs to be replaced.

About handlebars, Deda have the same diameter as the other manufacturers and it's around 31.75. The difference is just that Deda round down and the others round up. There's a different thing though - I'm guessing you're looking into Deda because they seem to have 46cm wide handlebars. If so, keep in mind that Deda measure outside to outside, so their 46cm is the same width as, for example, 3T's 44cm.