if you backup to disk (file library or storeonce software store) you also need to set the media pool to "non-appendable".
for mulitiplexing to a physical tape. here you can say about concurrency "use as many as needed but as less as possible". keep in mind, more concurrent stream for backup can increase the backup speed, but it does slow down the restore speed. normaly i start with 4-6 streams. if this is enough to keep the tape in streaming mode (Data Rate Matching for the Device) then i leave it like this...