An army belongs to the King as well as the Kingdom. Scottish regiments participated in wars, the same as English ones. I don't believe that England was paying for everything although I can't say with absolute certainty.
Now the higher the revenues the greater the tax burden and since Scotland had fewer revenues, it probably generated fewer taxes. But this did not exempt them from taxes to the Royal coffers or fielding an army when the King called them to service, the same went for the Hungarians, the Lithuanians, the Czechs, etc...
Note though, that the countries you mention still had their own administrations, and the King was bound to different laws in each country, most of the time. The Habsburg Empire is a very good example of this; it was not "The Austrian State" - more like the property of the Habsburgs. The decentralized nature of personal unions, and the
lack of control for the senior "state" - a name that is very ill-fitting for a feudal society, incidentally - is why acts of union exist. Despite sharing a single king, personal unions were still unions of politically distinct entities, different from real unions (a distinction EU4 does not represent) and certainly political unions (which are sufficiently represented by Integration).
Mind you, I actually disagree with the idea that PU armies should be controlled by the senior partner as a concept - the "control for pay"-suggestion is mostly a thought experiment on how I would personally balance such a feature if it were to be implemented at all. While it's true historically that yes, the army belonged to the King, the matter of feudal armies is
extremely poorly represented in EU4. It does not have the mechanics to support non-professional armies. Even mercenaries are abstracted to be virtually unrecongizable from their historical reality and function. I think the current system does an adequate job of representing personal unions as being the decentralized systems they were, and integration of them as a sufficient representation of a political union that would centralize the two entities into one more state-like.
Incidentally, if we're going to use feudal systems to argue for control of armies, we really ought to be clamouring for
less control, even of our own armies, not more.
