S/Os: SO is a rank, my company has S/Os but they are co-pilots in their own right and log P1 time as any F/O would. I take it you mean relief pilots as Qantas has.
If so, they don't handle the aircraft below 20,000' - depending on national regs of course. They tend to do twice as amny sims as other pilots (checks every 3 months) and some companys give them extra training as well.
It's a soul destroying job (according to my mates who have done it). Best chose an airline that doesn't have them if you enjoy flying.