As for the Dmin pent scale over the G7. The notes of a G7 are G, B, D, F. The notes of a Dmin pent are D, F, G, A, and C. If you take those scale notes and relate them to the G7 chord you get D (the 5th of a G7 chord), F (the 7th of a G7 chord), G (the root of a G7 chord), A (the 9th), and C (the 11th). In playing a Dmin pent scale you are using some notes that work directly with the chord (D, F, and G), but you also have two other notes – A and C – that makes your solo a bit more interesting. The 11th (C), typically, is not something you would really try to emphasize because it clashes a little bit against the note B of the G7 chord, but playing the 9th (A) sounds really cool.

To your last question, there are essentially two schools of thought on how to improvise over chord progressions. One school would tell you to play certain things over each chord. This is thinking vertically, and when chord changes are happening very quickly then you really have three choices – play very few notes over each chord, don’t play anything, or play what you were playing over the chord without really addressing the chord itself.

There is another school of thought where you work horizontally – with the progression itself. In this case you are less concerned with a G7 as much as you are working against a 4, 8, or 16 bar phrase. So, the G7 may be part of a CMaj7, A7, Dm7, G7 progression, in which case you approach all four chords as one unit rather than four separate ones. You can add color tones throughout and you end up focusing more on the entire phrase rather than breaking it down into separate chords.

I hope this helps!