The "rule" about parallel fifths/octaves does not say that they're "bad" or that you can't write them. What it really says is that if you have two independent voices, and those voices have a spot where they move together in a parallel fifth/octave, then we tend to hear those voices as sort of "linking up" and losing their independence for that moment. This shift from independence to dependence and back can be jarring. It's also very unidiomatic for music written in (or in the style of) the common practice period (Baroque through Romantic eras).
If you're using a vocoder to duplicate the sound, then that duplicated line is not independent, and the "rule" doesn't apply.
If I backwards 7 semitones from C1 and play F0 - is that also a perfect 5th?
How bad would F0 C1 G1 sound as a harmony?
First of all, it's up to you to decide how it sounds. Do you like it? Write it.
Second of all, this is called quintal harmony, and it's not very common but it's been explored in modern music.