Paul,
can i put a 1 GB harddisk in a Mac SE???
Since the hard disk is smaller than the maximum volume size allowed (2 GB before System 7.5), it should be possible to do this (without partitioning), provided that the dimensions and the connector are OK.
A word of warning: Working inside a compact Macintosh computer can be very dangerous. High-voltage charges may be present, also when the machine has been shut down and disconnected from the mains. Glass may break. Such work should be carried out by an expert.
As an alternative, the hard drive could be placed in a suitable case, connected to the external SCSI port.
Please note that Apple HD SC Setup is intended for Apple hard drives. For third-party drives, or for Apple hard drives that have been reformatted by a third-party utility, another program will (normally) be needed. Drive Setup 1.7.3 can handle certain non-Apple drives. Otherwise, look for FWB Hard Disk Toolkit, Lacie Silverlining or similar utilities.
The 8100/80 is not a bad idea. Both machines can be part of a LocalTalk-based network (or just directly connected, by means of a MiniDIN-8m to MiniDIN-8M printer cable; see
this page for details).
Jan