For many applications aluminum rivets will not be strong enough and stainless are required; for example, tangs for shrouds.
When mating aluminum and stainless steel it's important to electrically insulated them. For large surfaces thin vinyl tape is good. For rivets, screws, and small parts, TEF Gel is best. Blue Loc-tite can be used as well.
For high-strength, critical applications I prefer Cherry Max stainless rivets. This is for things that will never come apart. For things that may have to be removed you can tap the mast and use a stainless machine screw. Higher strength applications may require a nut on the other side.