I want to replace these 7” speaker holes with recessed lighting. Are there any issues with using 6” cans and have a 0,5” border opening? I’ll be using the new-construction cans to nail to the wood instead of the remodel version so anchoring isn’t a problem. Also will be using 8” trims. Would dust from the attic be a problem? Or something else an issue?
No problem at all doing this. But I would come up with a way to fill in that gap you have. It will allow drafts and dust penetration. Personally, I'd square the opening in the ceiling, take a new piece of drywall the same size as the hole and put the hole for recessed can in it and then secure it to the existing ceiling drywall (used 1/8" plywood for securing the screws into), mud, texture, put the can in (new or old work), paint and then the trim.