Harold put a vassal in place in Gwynedd in 1063. But IIRC they were already ruling and had originally taken Gwynedd by force with an Irish & Danish army.
Re counties, there were established principalities in the West of the British mainland -- Gwynedd managed to subdue many of these other counties (ie independently governed areas), whilst paying fealty/homage to the English Crown, once Gwynedd had subdued Deheubarth and Powys and such, they (Llewellyn The Last) stopped paying the Crown. At which point Gwynedd was put down, handing their recent won territories over to be annexed to the "English" King's territories (Edward I).
I don't think the Welsh counties in general were divided in to hundreds until Henry Tudor (who also issued a writ demanding use of English across his whole territory). Hides and hundreds, like the words, are Anglo-Saxon I think?
AFAICT Gwynedd grew strong (post-Romans) by earlier conquest by Vikings and Irish-Scots.
You're right, the Norse, like the Romans before them didn't care what tribe you hung with, Wales is called that because it's the bit the Romans hadn't conquered yet (cf Walloons, Wallachia (? in Romania)); which is down to physical geography. Same with the Norse AFAICT. And the same in the other direction with why Gwynedd was able to conquer the principalities but not subdue the Marches or Pembrokeshire.