If two devices have the same ID number, you can see all sorts of problems and crashes. There are some standard conventions for numbering. ID 3 is used for CD-ROM drives. ID 5 is common for Zip drives and ID 6 for scanners or tape drives. Keeping to the conventions can help minimize problems as some devices come pre-configured.
Some devices have internal termination that can be turned on with a switch.
All SCSI devices connected to the machine should be turned on while the computer is in use. Lots of people ignore this guideline, until they have problems. There isn't a strict rule about the order in which to hook up SCSI devices in a chain, but there are some guidelines. Fast devices that transfer a lot of data should be put closest to the Mac in the chain. Scanners and tape drives like to be last in the chain.