Hi tcafe,
yes all root aggregates have to be mirrored.
As you stated you can use a shelf with smaller ssd's to host the root aggregate on it (please check the ssd limits that the platform supports prior to attaching disks).
system node migrate-root works in 4 node MetroClusters and yes you can move the root aggregate there, in 2 node MetroClusters in is a slightly different procedure but that one also works.
When moving the root aggregate to smaller disks you also need to respect the minimum root volume size for the platorm so in the unlikely event a core file could be saved.
Thx,
ak.