Barbs should to be in groups of six or more to prevent nipping and bullying of other fish and eachother. They school and display better too. So actually no, not quite as stupid as you may think, the last thing you need with a betta is a fish with such tendencies . Six fish of that size in a 10 gallon plus a betta will be cramped, even more so if he want's cories as well.
It would work a lot better in a planted 20 gallon. He could seal off a 5 gal section for the betta and have the barbs or other fish in the other section.
Threadstarter: I notice now you want oto's and cories AND harlequins in with the betta. Yes those would work in terms of temperament, although 10 gallons is pushing it a bit for space. You could possibly get away with 4 pygmy(dwarf) cories, 2-3 oto's and 4 harlequin rasboros. Although the rasboros do seem to be better in bigger groups, and look lovely when they school. But it could work I think.