Yep, if the main pool is redundant, then the special device should be as well, or the whole pool dies.
You can make a mirror deeper, by attaching extra drives to it, so like 3 deep will have the capacity of just one device, but any two of them can fail at the same time, or one can fail, and while waiting for a replacement, another can fail, and you still have a working pool.
I don’t think you can raid special devices (not sure that is even on a roadmap or not)