Sponge8.0，8.1 plug-in, can be used on Sponge8.2.1？
Sponge11， plug-in, can be used on Sponge8.2.1？
To your first question. Yes
Essentially, plugins should work if the number before the . matches with the rest being the same or lower then yours
As for a api 11 plugin working on 8, no. If you were running a api 11 server and wanting to run a api 8 plugin there is a chance of it working but not if the plugin requires a newer major version (first number)
Heres the official docs on the topic
https://docs.spongepowered.org/stable/en/versions/filenames.html#spongeapi