Hey, this should not be a problem. We had a STK silo (acsls system) with 9940B drives and 3590 drives.
Basically, define another library definition using the same library device (whether using acsls or some type of scsi library). So, create another logical library name.
Then define these new drives to this new library, then build a new device class, and new storage pools on top of that. Notice how this will be a complete different structure and no mixing. So when you do the labels and checkout/in you will reference the new library name. You can still use the old tape/library, just make sure to reference that library name when doing the apprioprate tape operations.
Rick