A couple of additional notes that might help answer your question:
The "blues scale" was developed to to play one set of notes over all the chords in a twelve bar blues, so the blues scale based on A minor pentatonic is played over a twelve bar blues starting on A7, even though the major third does not technically occur in the scale (this is why chords with a sharp 9 sound bluesy). In general, either minor pentatonic or the blues scale can be played over dominant 7 chords to achieve a blues feel, although this may not be appropriate depending on the context.
More generally the pentatonic scale, in it's various inversions, can fit over a large number of chords. A good rule of thumb is that whatever key you're in you can construct melodies using the major pentatonic scales starting on the first, fourth or fifth degrees. Notice that the blues usage of the minor pentatonic actually falls outside of this rule, and thus provides a fourth possibility.
To answer your final question, associating scales with chords is a complex subject that can vary with different styles. Typically, though, it depends on the function of the chord, so if a Dm7 is being used as the ii in a ii-V-i you'd play dorian, which would be the same notes as the C Major scale (so, in pentatonic terms, you could use notes from C, F, or G pentatonic).