What kind of modulation is V in E♭ harmonic minor to V in C harmonic minor? Are all V to V modulations acceptable regardless of the key? Or are not all keys possible to use for this progression?
Also, does there have to be harmonic to harmonic or can I do the major key to minor key V progressions?
That appears to use a secondary dominant into a decptive progression in cm to prolong the dominant(Bb). So the G could be written V/#vi. There doesn't appear to be any actual modulation or even tonicization since there is not a strong cadence in the key of cm.It does hint at mode mixture to the parallel major (Eb maj) with the major plagal progression IV I in the dominant prolongation before the last Bb. In short it flirts with cm but never makes it there. So you could think of the progression from Bb to Bb as keeping the same overall function of Bb through the section but making it more interesting instead of staying on Bb the whole time. To answer your question about Bb to Gb - they have a chromatic mediant relationship. You should be able to find more information about chromatic mediants on Wikipedia or near the end of tonal music theory text books. They usually involve chromatic movement in their smoothest voice leading.
