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I have doubts about some chords in this sequence:
||: Eb7 | Ab7 | Eb7 | Eb7 |
| Ab7 | Ab7/A6 | Eb7 | C7 |
| F7 | Bb7 | Eb7/C7 | F7/Bb7 :||
Harmonically it is a blues in Eb7 with progression I-IV-V.
I suppose that C7 is the secondary dominant of the second.
What would be the correct way to analyze A6 and F7?
In general, could be use both the minor pentatonic and the major pentatonic?
With blues, both major and minor pentatonics can be and are used all the time. Even better, with blues (and it makes sense!) would be to use major and minor blues scale notes. I'd go even further and advocate using chromatics too, mostly as passing notes, so basically, any notes, almost anywhere, if they sound good, will be the order of the day.
That said, here, there's partly the circle of fourths going on. Yes, it's in E♭, but moves to C, then up 4 to F, then up 4 to B♭, then E♭, and goes round again. As it's doing this, a lot of players would 'play the changes', and use each chord to provide certain chord tone notes that fit more exactly over each chord.
A to E♭ is a tritone interval, and that's one way to explain it theoretically. Another could be that the A is a semitone above the A♭ just before it. It's just an effective change - I last played it in 'Blues in the Night', but it went back to A♭ straight after.
Often there isn't a straight explainable reason which could justify 'theory', but as we often say 'if it sounds good, that's good enough'!
That is a pretty straightforward blues progression with a couple of embellishments. You could think of the C7 as a secondary dominant, but I would rather think of this as part of a progression leading back to the Eb7. This C7 marks the beginning of a VI7 - II7 - V7 - I7 progression. The progression is repeated twice, and the second time it is used as a turnaround leading back to the beginning of the form. In fact, this is a common progression to use for turnarounds in jazz and the blues.
As for the Ab7 - A6 - Eb7, the A6 looks to be a chromatic passing chord, so I would just analyze this as IV7 - #IV6 - I7. Notice that A6 and Ab7 share a note: the 6th of the A6 (F#) is the 7th of the Ab7 (Gb), taking these as enharmonic equivalents. The Eb7 and the A6 also share a note: the 7th of the Eb7 (Db) is the 3rd of the A6 (C#), again taking these as enharmonic equivalents.
Instead of A6, you could play an Am6 here, or better: an Adim7. A common variation on this would be Ab7 - Adim7 - Eb7/Bb. Here you play the Eb7 over a Bb (i.e., in second inversion); this gives a nice bass movement leading to the following C7 with Ab - A - Bb - C.
