I'm equipping a Beseler 45MXT for medium format, especially 6x7 and 6x8. Can I use a neg carrier for 6x9 for both, or do I need separate ones for each size?
You really don't even need to mask it off. Yeah, light spills over and could in theory fog your highlights, but I've never seen it happen in practice. The bigger issue, I think, is film flatness.....but even that is probably not all that big a deal from 9 to 7 cm. I'd say try it and see whether you are happy with the results.