They make mini balltops for sanwa and seimitsu sticks, so that combined with the 24mm should be good.
I personally don't think they would be good for six button fighting games, but if all you are after is a test rig, then I think they would be fine.
However, I think you should go with 3 regular sized buttons and 3 small test buttons for each player. This way, you can play 3 button games normally while preserving the ability to test 6 button games.
What kinds of buttons are smaller than 24mm? There has to be some very tiny buttons for non-arcade use that he could put in.