I would go with Trigonostigma Espei's instead of the Harlequin's in a 10 gallon tank. It would be more enjoyable to watch. You could do 8 Espei's and 12 Chili Rasbora's.
If you go with the Harlequin's or Neon's, my opinion is that a single school would be full stock. You could do 10.
The answer for your question about the minimum school size would be 8 to 10.
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