... Iím going to rivet aluminum over steel. Wonít I get some kind of fast deterioration from the incompatibility of the two metals?
How do I protect the aluminum? ...
Without weighing in on the aluminum-vs.-steel discussion, the deterioration you are talking about is called "galvanic corrosion" and, yes, it can be a real issue if any moisture comes in contact with the assembly.
FWIW, we re-skinned the roof on our otherwise steel bus with aluminum. To avoid galvanic corrosion we used butyl tape between them and butyl-sealed "bulb"-type rivets. Then we painted the roof. In seven years we have not noticed any issues with galvanic corrosion in the roof area.