I have purchased a SCSI XLerator, do I need ANOTHER SCSI card or just SCSI to Ethernet/Wifi?
You don't need a SCSI expansion card if your XLerator has SCSI installed.
If you were to install a SCSI expansion card as well, you can use only one or the other SCSI port. One uses the XLerator control panel to turn on/off the XLerator SCSI port (if it is off and a SCSI expansion card is installed, then that card's port is used).
Note that SCSI support was introduced in MacWorks Plus 1.0.something and is in MW+II. SCSI isn't supported in LOS, Workshop, Xenix, MacWorks XL as there are (currently) no drivers.