To me having the floor at 1 SP rather than 1 GP just makes everything feel more believable, and I think they could peg the actual conversion into GP there at whatever ratio made the most sense for the gameplay balance.
I agree. Finding silvers and coppers, if implemented, would be more common than finding gold.
So in D&D terms, if you had nine silver coins and picked up two more, it'd display as 1gp 1sp. No need to go into elaborate inventory tetris any more than we already have in game with stacks of different currency taking up space.
I couldn't really see it working like this unless characters go to a bank, miner's exchange, or merchant who is willing to trade silver for gold at 10:1. I can, however, say that it might still be realistic to use silver to pay for things that cost gold, provided the player has enough. For example, buying a goat for 1gp would cost 10sp if the play had no gp available. It's really just a matter of personal taste. Perhaps automatic transference should be an option in the difficulty settings, if implemented.
