as far as my understanding Dal Riata and Alt Cluit had separate rulers between 508 and 540.
and for genealogies i would rather put my trust in the
Harleian genealogies rather than
Bonedd Gwŷr y Gogledd. the latter seem more like propaganda to boost the prestige of certain royal houses.
the institution of Ard Ri or High King was maintained in
Ireland(at
Tara) and possibly
Pictland(at
Scone) since ancient time. the Britons seem to have re-established the title in some form after the roman departure, possibly based on structures set in place by Magnus Maximus. at any rate the
high kings seldom held any real power outside his own domain, i suppose it was mostly a token of prestige. of course some persons, like Vortigern, might use the title to gain actual political power.
but the title probably lost its importance with the decline of the central government as Britain gradually split up into petty kingdoms. the last holder of the title, as to my knowledge, was Cadwalader king of Gwynedd that died of the plague in 664. by that time the title was a sorry remnant of what could had been.
even the anglo-saxons had some sort of high king in their
Bretwalda.