With no other tankmates, 6 otos would be fine. However, some things you should note:
- in future, try to avoid putting zebra danios in anything under 2ft tanks. As you have probably noticed, these fish are ridiculously active and really appreciate the zoom-room in larger tanks.
- otos are wild-caught using cyanide. This makes them very delicate and they are not suitable for tanks that are new, heavily stocked, unplanted or only lightly planted.
- otos are schooling fish and really should be kept in groups of five plus. Numbers = security = less stress = healthier fish.
- stocking bettas and danios together is hit and miss. Danios are subtropical and prefer cooler (68-75F), faster moving water, as is found in their native rivers. Bettas prefer tropical (78F+), slowmoving or still water. Bettas can also be stressed by the fast moving danios, which can also nip. If you can avoid putting them together, I would. :)