I wouldn't sweat the 1 meg. If you've got a Fenderbucker and you're mixing raw pickup outputs with no tone controls, odds are a lower value pot is going to keep the mids and top end from getting weird and phasey anyway. I'd use a 250/500 for the first two pickups and a 250/250 stacked knob for the other two. I'd wire the 500 to the bucker to give it a more useful taper, and put the 250 on the s/c. If the s/c ends up being too muddy (which I doubt it will), then try a 500/500. That bass will be a monster.