Well, just from the number of fish I would think the tank is a little overstocked. Get a slightly larger filter and do 50% Water Changes weekly to make up for possible overstocking. Personally I would do that even on an understocked tank, as it's always healthier for the fish if you overdo it instead of under do it.
The stocking is fine, those are all tiny fish, with small bio loads. I would just stick with 3 otos those guys would be just fine, you could probably bump up the number of pygmy cories too. Personally i would ditch the betta and add some shrimps.