Colombia is a really good neutral stage. It contains platforms, which do help for some characters as well as large open areas. My only qualm with this stage is that the slants interrupt some characters' combos (Most specifically BD, who really benefits on flatter stages). I really like small Practice 2 (I believe that's the platform one) as well.
Failing that, Practice Medium is good too.
While I like Hades, it is not a good neutral. Maybe a good counterpick, but not a neutral. Camping below those platforms is really effective.
Oh and Dojo is a TERRIBLE starter. It's really really small in the first part and in the second part it has no walls allowing some characters to set up really stupid combos. It's a good counterpick, but that stage is in no way neutral.
I think any of the Practice Stages are good for competitiveness. Of course, don't take my word for much, I've still only been closely watching others play and waiting to get the game. It seems to me though that Hades or Columbia would also be a nice choice, though.
I would have to say hades, not too small or big. gives gun characters some stretching room, but combo characters can also thrive. the stuns also change it up a bit as you have to stay on your toes at all times.