IIRC, 25 pin and 50 pin SCSI is identical except at the termination side. half of a fifty pin connector is used for grounding anyway. if you like i can (laboriously) give you the pin outs for most kinds of connectors and terminators, but only if you REALLY need it.
If you have a 25 pin connector on your card and the scanner, it'll work. If you have a 25 pin on the card and a 50 pin centronics on the scanner it'll work. If you have a 68 pin on the card and 25 pins on the scanner it'll work. most modern cards are auto-sensing for this and change the pin outs and speed appropriately.
Having said that, it is a rare form of voodoo to get all the combinations working correctly. As long as you have similar types and turn on the peripherals (scanners etc) before booting the computer you're cool.

Being a bus, terminators must be at each end. Usually the card has termination by default. The exception is where there is an internal connector and the card is in the middle of the chain. If your scanner has an internal terminator, it must be at the end. make sure that the IDs are all different and you should be fine.