some educated speculation

I'm prety sure to hot-swap like you're describing you'll need drives with an SCA interface (80-pin connection that carries data and power). There might be drive enclosures that let you hot-swap normal 50 and 68 pin drives - it's been a while since I've set up a SCSI system. If you want to be able to swap all the drives you'll probably need a SCSI RAID controller to have redundant storage - as far as I know, plain SCSI controllers don't support hot-swapping.

I think mirroring would let you remove even the boot drive, but if you have 2 copies of each drive, you'd need to make sure not to remove both copies of a drive at the same time.