How to use LTO6 and LTO8 Drives simultaneously

The one exception is qcart - do not ever use this ...

All the other densities are just labels, hcart2 tape goes into an hcart2 drive etc ...   qcart is a special case and causes NBU to write in a fixed block size, which if you are not using qcart drives will cause failures.

And use different barcode series for LTO6 & LTO8. Makes it a lot easier to manage and identify media

e.g:
A00000-A10000 LTO6
B00000-B10000 LTO8

If you go with Nicolai suggestion of "A" and "B" media, then here's how I might configure a new environment, but you might not need all of this, as it sounds like you already have some parts of this configured, so you'll have to pick out the bits that you do need, and/or modify accordingly as neccessary to suit your site specific needs :

vmpool -listall -b
vmpool -list_scratch
vmpool -add         ScratchPool "Scratch pool" ANYHOST -1 -2
vmpool -set_scratch ScratchPool
vmpool -listall -b

vmquery -list_media_genrule
vmquery  -add_media_genrule 0 8 "1:2:3:4:5:6"
vmquery -list_media_genrule

vmrule -listall -b
vmrule -verbose -add "A"    hcart3       ScratchPool  0 "LTO6 media"
vmrule -verbose -add "B"    hcart2       ScratchPool  0 "LTO8 media"
vmrule -verbose -add "CLNA" hcart3_clean None        25 "LTO6 cleaner"
vmrule -verbose -add "CLNB" hcart2_clean None        25 "LTO8 cleaner"
vmrule -listall -b

# ...inventory the robot and list media present in robot:
vmquery -b -rn 0
vmupdate   -rn 0 -rt tld -use_barcode_rules -use_seed
vmquery -b -rn 0

.

And the hcart "types" I use above are derived from... typically in the past, one would have expected to see hcart types like this :

LTO1 hcart
LTO2 hcart2
LTO3 hcart3
LTO4 hcart
LTO5 hcart2
LTO6 hcart3
LTO7 hcart
LTO8 hcart2
LTO9 hcart3

...i.e. one would not normally expect to find a library containing LTO drive types more than three generations apart.