you can't install the 7.6 inventory templates in 8.x - you'd want to use the 8.x templates anyway since some of the stuff in the 7.6 templates isn't used by bdssa anymore.
the problem i think you will have is that the older CO versions from 7.6 are not present in 8.x nor is there currently and easy way to import them, so the snapshot of the various COs will not work i think as you found.
is there a reason you can't upgrade the agents ? it's fairly easy to push out agent upgrades: https://docs.bmc.com/docs/display/public/bsa83/Upgrading+RSCD+agents
if you get everyone to 8.2 to match the appserver and then upgrade this env (keeping the past COs) then you won't have an issue.
this issue has come up before so you may want to open a support ticket and see if there is a way to import the older COs - depending on how newly installed your env is, there might be something possible.