I did this tour precovid and the single rooms were fine, in some instances as big as the double rooms.
Rick's tours have a general disclaimer on the possibility of a shared bath situation but the place to find that is on the overview for each specific tour. The Scotland tours do not have any shared bathroom situations. The 21 day Best of Europe does have that chance especially for the single room participants. Here's where to look. Go to the tour page, scroll down to "Physical Demands" and click the plus sign to open it. Under 21 Day BOE, it says "Be content with the possibility of sharing bathroom facilities at one hotel during this tour."
https://www.ricksteves.com/tours/europe/europe-21-days
No need to go down the hall for Scotland!