Is a dim7 chord pretty much the same thing as a flat 9 chord a half step lower? Like C#dim7 is the notes C# E G Bb... which would be like a C7b9 without the root? Can these chords be used pretty much interchanably? Like, similar function, little bit different sound/feel?
yes. although i think the comparison is a bit more solid if you call it Edim7 (same notes anyway) and C7b9. Edim and C7 are sister chords. just like A7 and C#dim