Inchworm is on the money with this not being an exact science, and often being harder to explain than to do.
Let's take your 20 long as a good example. If each tetra counts for 10% of the bio load, that gives you 80% with all eight together, in that tank. If the plec counts as the rest, 20%, that adds up to 100%, in your 20 long.
If it's a bigger plec, it may be 40% of yor bioload, making each tetra 7.5%. No matter what, the fish in your cycled 20 long will add up to 100%, no matter how much or little of the bio load they are.
Each fish is using it's percentage of the filter media in the tank. If it's a bigger plec, using 40%, you can move the plec & 40% of the cycled filter media from your 20 long to another tank. Smaller plec, go with 20% of the media.
The non-exact part is figuring how much of the bio load on the cycled filter each fish is. It would be easy if we were just talking about the eight tetras, which are probably around the same size. You could take half of them, & half the cycled filter media, add them to a new tank, & be just fine, or two & 1/4 of the cycled media.
It gets a little tougher when you have a situation like yours, with 8 smaller fish & 1 larger one. You have to more or less eyeball the fish, & try to estimate how much of a percentage the fish you plan on moving are out of the cycled tank they are in.
Once again, being non-exact there is room for error. If you figure the plec at 40% of the bioload when it is actually 30%, move 40% of the media & the plec into another tank, the 20 long with the 8 tetras will be fine.
Same goes if you figure the plec at 20% when it is actually 30%, move 20% of the media & the plec to the new tank. Since it is well matured cycled media it will grow quickly to make up for the difference.
HTH, and like Inchworm said, keep asking until we figure out a way to explain it correctly.
Tolak