Most inner duallys are steel for cost saving and that they are thinner than aluminum wheels. When rotating, the inner wheels usually only go side to side on the rear. If the Seneca has 6 aluminum wheels, it is not for weight savings.
I have 4 aluminum wheels that I am redoing to replace the steel ones on my Greyhawk. I will definitely be keeping the inner steel wheels. I'm not sure if I would have to replace studs for longer ones if I went with 6 aluminum ones. I haven't measured.
As for the two dissimilar metals, it's a non issue. Every aluminum wheel is bolted to a steel hub. It makes no difference if it's a steel hub or steel wheel it's bolted to.