I banged rivets on a production line long ago. The countersunk rivets used in the pressure hull depend on expansion of the shaft, but some rivets also have a slightly larger radius where the shank flares out into the head compared to a standard flush rivet, which forms a tighter seal when the rivet is set.
You don't actually apply sealant to the rivet itself. The sealant, which is a crazy-sticky goo stuff (not epoxy, it cures to a hard rubber consistency), gets everywhere when you are shooting rivets on the pressure hull because the faying surfaces (the overlapping metal to metal surfaces) are coated in sealant before assembly, which gets in the rivet holes (the sealant is a pain in many ways; it acts like a lubricant if it gets on the skinny end of the rivet and makes it easy to dump the rivet over when you buck it).
Carbon fiber panels are usually joined to metal with bolts or Hi Lites or similar rivets, with some sort of insert or doubler of stainless or titanium in the carbon to distribute the loads. Sandwich floor panels for example, typically use a titanimum insert bedded into the sandwich with epoxy resin, through which the fastener goes. Carbon against titanium is fine, but if joining carbon to aluminum, you MUST have an insulating material to separate them because they live at opposite ends of the galvanic scale. Carbon touching aluminum = lots of corrosive fun.