It's not overstocked, but the advice is poor, as Neons and Otocinclus need the water to be cooler than a Betta prefers. Neons can also be fin-nippers, so they may nip at the Betta's fins, and Bettas are potentially quite aggressive and may bully the other fish. Usually, we'd advise that a Betta be kept alone in a 40-litre tank.