If you are worried about the strength of aluminum rivets, just think about the load directions they will see. If you are just holding spreaders into place so they do not fall off when not under load aluminum will be just fine. Another example is holding a cast aluminum cover on the end of the boom.
If the rivets will see substantial sheer forces, then you want to think about stainless or monel. Monel does not react with aluminum so you do not have to worry about isolating the rivets. If you use stainless or monel, I suggest getting a pneumatic pop (blind) rivet gun. It will seat the rivets much better than a hand powered one and they are not too expensive.
Monel rivets are expensive unless you buy them by the thousands. So, expect to pay around $0.50 for each rivet.