not sure really. i just find it easy to rest your 2nd finger on the low E string round about the 9th fret. after all, its not doing anything else when you use that shape. You can mute the E string with your index finger, but I personally find it easier this way, and i think its worthwhile giving it a try.
to play the F on the g string on the d string, then you'll have to fret the 15th fret, which means you'll have a impossibly large gap between the 8th and 15th frets. you can play an A octave by playing the A string open and the 7th fret on the D string.
It might sound hard at first but its a really easy way to strum octaves fast without hitting other strings accidently. try using that shape, making sure that the strings that are meant to be muted are muted, and the F notes are ringing clear. Then you could start on the 3rd fret (C) and go up the C major scale C D E F G A B C using frets 3, 5, 7, 8, 10, 12, 14 and 15 on the A string:
C D E F G A B C
E x--x--x--x--x--x--x--x--|
B x--x--x--x--x--x--x--x--|
G 5--7--9--10-12-14-16-17-|
D x--x--x--x--x--x--x--x--|
A 3--5--7--8--10-12-14-15-|
E x--x--x--x--x--x--x--x--|
Check out Dave Navarro-era RHCP. The album One Hot Minute has loads of octave solos like this, for example "My Friends" and "Tearjerker". Loads of bands use octaves though, as its a good way of adding depth to a melody.