I think I get it. Ignore the high amperage in rush of the starter and use a slow blow fuse rated at around the starter's constant draw. Maybe even at 150% of the normal draw.
And...what happens when you put the battery switch to combine and your house batteries then could provide a continuous 2-400 amps to the starter (or whatever you have it fused at, but probably higher than the battery fuse) but the cable to the starter only has the ampacity necessary for what you thought would be the normal starter draw. Of course its not a problem under everyday circumstances but with a starter problem you could overheat that starter cable. Seems like the cable should be sized for that potential, however unlikely. I suppose just the cable from the start battery to the battery switch could be smaller. Or maybe having a fuse at the start battery and another on the starter side of the battery switch would do it too.
I think the current recommendation is to fuse all circuits, even those to the starter. Blue Sea has just come out with a terminal fuse that makes it a snap to add. Its not on the BS web site but its in their 08 catalog or you can visit Jack Rabbit for a view.