1) Depends on the SCSI controller and the scsi.device, I believe.
2) Probably not "too fast" but certainly can be incompatible. There's a lot of revisions of SCSI, but most likely, you'll want a drive specifically for legacy SCSI applications. Taking a high-end server HD and putting it in an Amiga isn't likely to give very good results. SCSI is a very diverse standard, and not completely compatible. I highly doubt an Ultra320 drive will work very well on an old SCSI-1 card. Of course, now saying that, three people will stand up and swear it works great for them. Still, I'd use caution when selecting a new HD.
3) copy clone source: target: all
I'm serious. That should work fine. It's what I've always done. Provided you formatted and installed the boot RDB, you should be all set.
