Jim,
>>That's my hope. I just wouldn't be too please to learn otherwise - spending the extra money for virtually no benefit. I'd have confidence with only the new faster drive attached.
I think the main problem you might face with the old SCSI drives is with the cable/drive type (Wide, fast/wide, ultra-wide, whatever) -- something to ask about, cuz I don't know the answers. However, some controllers are "dual-channel", probably meaning that they have 2 SCSI connectors, allowing 2 cables -- one to the new drive, and one to the old ones. That would probably work great.
John is right that they work pretty much independently. I would ask, however, about the controller settings for maximum speed (20 mbps, 40 mbps, etc). That MIGHT have to go to the lowest common denominator unless it can be set independently for each drive or channel. In that case a second SCSI controller would work well.