[Zeke]

Generally fireboxes and all apparatus require 1" of clearance all around (you need the installation specs). There are collars for the vent pipes that should seal that 1/4" you have. The vent should be double or triple wall so the heat transfer through the collar won't be a hazard.

I would not stuff it full of rockwool. Supposedly it won't melt or burn but you do want the vent pipe to be able to radiate some heat. There may be some silicone that can be used. For sure I'd consult the installation instructions and keep it to code for any insurance issues.