I am no expert, but with all the rigging hassles I have been going through over the last few months, I am quickly getting there
When you mention "lower shrouds" I am presuming you are refering to the inner stays???
"upper shroud" goes to the highest point on the mast, depending on rigg type, but often it goes to the mast head
"lower shroud" follows this one, goes through the first spreader and terminates to the mast at the base of the second spreader.
"inner stays" usually terminate to the mast below the first (lower) spreader. The base termination is fore and aft respectively.
Inners are their to support the lower mast level and to control bend in the lower mast area slightly. They can be much lighter in size compared to the shrouds as they don't carry the same loading. There is no benifit to a heavier gauge wire to be used here. If one of these wires ever break, you are up the pervibial creek in a barbed wire cannoo with out the paddle. In other words, the rigg would have gone over the side anyway. In the event that you did lose one of these and only one of them, then a temporary repair of an inner with a heavier wire gauge is fine anyway.
As already mention, where do you stop with spares. You have to draw a line somewhere. Or we would all end up towing a barge behind us for spares.If it got to that stage, we may as well tow a spare boat. It's what multihullers do you know, multihullers are just parinoid mono sailers that carry along a spare.