By the standards presented by the game for what Ultra Beasts are, it would be very much possible to scale Solgaleo and Lunala (along with the other UBs, probably) to all other non-god tier legendaries in the main series, since beings from Ultra Space are not only stated to have different boundaries than those in the regular universe, but specifically to exceed them. Little things like this are often stated via things that on their own could be argued to be hyperbole, but in context (the result of extensive scientific analysis by global police and scientists), are likely all true.
For instance, UB-02 Absorption (aka Buzzwole) is said to have a punch that would be "off the charts" and impossible to measure. This is coming from the same people who can analyze the strength of things like Machamp, Tyranitar, or even pokemon like Regigigas. UB-04 Blade (aka Kartana) is said to have an odd body structure which, while paper thin, has blades so sharp they can cut through any substance currently known. UB-05 Glutton (aka Guzzlord) is eating every single second it's awake, and converts all of this to energy which it absorbs and uses to fuel itself, resulting in no waste whatsoever.
Of course, this does not show that the UBs + Solgaleo and Lunala are star level, but it shows that even compared to other powerful Pokemon in the franchise, their power is treated as horrifying and incredibly dangerous, and that they all get pretty solid scaling despite many lacking proper feats of their own. The Tapus would scale as well, but they are obviously weaker to an extent (it took all four to stalemate Solgaleo/Lunala, individuals had to stay on the defensive when confronting random UBs, who again, are not even stated to be explicitly powerful members of their species/group).
