Because your boom is aluminum, using aluminum rivets will prevent the galvanic corrosion that can occur between aluminum and steel. Another consideration, the aft rivet has to go through the end plug, the outhaul fitting, and the boom, which necessitates a longer rivet. APS sells them either alone or in a kit: http://www.apsltd.com/c-2224-laser-sparsandfittings.aspx
I'd use monel rivets with corrosion inhibitor (zinc chromate paint otherwise known as yellow gunk). Aluminium rivets in my opinion aren't strong enough for the direction of loading these rivets experience.
replacing with larger rivets works well, sailed for years after replacing them,
I have used both aluminum and some heavy duty car rivets (zinc plated) I got from my mechanic friend, both worked without any problem
used the heavy duty ones on a boom that was badly corroded,
if you install correctly the bigger rivets will outlast the boom
You'll need to borrow a marson "big daddy" rivet gun for bigger rivets
Aluminum is not strong enough. Stainless or monel rivets are the go. Some people thru bolt . Before fancy Lano Cote and other stuff, riggers and mast guys often used white lithium grease to prevent corrosion between stainless and aluminum.
My friend's(composite & mechanical engineer) custom 32' race boat built in 1982 had spars that were drilled and tapped. Every fastener came out with ease when we resprayed the mast 25 years later. They were mostly 1/4-20 stainless(perfect for galvanic corrosion). White lithium grease is dirt cheap and is available at most auto shops.