I can answer some of those; others will very soon.
All can boot from an external scsi drive. I've booted all of those machines from external hard drives and zips, and many from CDROMs (I've not had any failures; I'm just saying that I haven't tried all of them).
One caveat: the Plus is a bit of an oddity wrt SCSI, because it implemented SCSI a bit ahead of the final standards. Faster HDs won't boot the Plus, unfortunately (and since faster drives tend to be larger drives, this basically means that you're limited in practice to booting from a drive whose capacity is south of 1GB or so, give or take a factor of two here and there).
There is no card slot in a Plus, Classic, or Classic II (the latter has an expansion connector, but it's not PDS). The SE and SE/30 both have expansion slots, but they differ from each other. The SE/30's PDS slot is the same as, eg., that in a IIsi, if that helps you at all. And '040 expansion cards for the SE/30 were made (although they are not easy to find these days).
All can boot from an external scsi drive. I've booted all of those machines from external hard drives and zips, and many from CDROMs (I've not had any failures; I'm just saying that I haven't tried all of them).
One caveat: the Plus is a bit of an oddity wrt SCSI, because it implemented SCSI a bit ahead of the final standards. Faster HDs won't boot the Plus, unfortunately (and since faster drives tend to be larger drives, this basically means that you're limited in practice to booting from a drive whose capacity is south of 1GB or so, give or take a factor of two here and there).
There is no card slot in a Plus, Classic, or Classic II (the latter has an expansion connector, but it's not PDS). The SE and SE/30 both have expansion slots, but they differ from each other. The SE/30's PDS slot is the same as, eg., that in a IIsi, if that helps you at all. And '040 expansion cards for the SE/30 were made (although they are not easy to find these days).