ally is the anode to stainless s cathode, making stainless fixings in ally sheet no problem, not enough to worry about anyway,
however if you reverse the roll and put ally rivets in stainless sheet your asking for problems as the larger the cathode to anode ratio the more corrosion problems
carlos has the best advice, isolate the two with sealer to prevent moisture getting between the two surfaces causing electrolytic reaction
unless you get a lot of salt water spray then theres no problem
stainless to stainless threads have a nasty tendancy to gall up and pretty much weld together [ a right bastard ]
usually a quick polish on a coarse buffing wheel to the threads works well
on some stainless tanks ive made with stainless caps which have to open often ive worked the threads together with autosol to smooth them. this works a treat, but you wouldnt want to do a ****load of bolts!!
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you get what you pay for if you dont youve been ripped off