It seems like you just make the seventh note flat, but are there any restrictions to that? Can any scale become a Dominant Seventh Scale by slight modification or does it differ on a case-by-case basis?
The Dominant 7th scale is one of the many named scales used in a jazz context.
It should be like the major scale, but with a flat 7th. From Jamey Aebersold's Scale Syllabus, it has the following pattern of whole and half steps: W W H W W H W (in C: C D E F G A Bb C). This is also commonly known as the mixolydian scale.
Many scales do have a flat 7th, but if it is not based on the major scale with the above pattern, it would have a different name.
Lastly, it is accepted to refer to a flat 7th itself, that is, the scale degree or note, as a dominant 7th.
The dominant seventh is only a chord and not a scale. For discussion on the different scale types, visit this wikipedia page.
To build a dominant seventh chord, take any major triad and add the flat seven on the top. In terms of intervals above the root, this is: major third, perfect fifth, minor seventh.
When considering scales rather than chords as in the ABRSM syllabus where dominant 7th scales are required, the dominant 7th scale consists of the major triad with minor 7th note but starting on the dominant note of the scale i.e. the dominant 7th of C is G,B,D,F or in other words, the arpeggio on the mixolydian scale of C. I found this out today while researching for grade 8.