I was thinking about making a song based on the F# diminished scale with a chord progression of F#dim5->Cdim5->Adim5 (all the notes of these chords would fall into the F# dim scale... I think). The problem is I think this could get a bit boring because as far as I can tell there would be only 4 notes throughout the whole thing. What is another scale that would work? F# Locrian?

Also, on a completely different note, I don't know the difference between G Ionian, F# Locrian, A Dorian etc. Is it just the notes you focus on more?
Try checking out the lesson on modes. It explains the difference, but quickly: it has to do with the intervals used starting at the root note. G Ionian uses the same notes as A Dorian, but its best to think of them individually (i.e. not as A Dorian as an offshoot of the G major scale). As for the progression sounding boring, play it and see how you like it. If it sounds good, it is.