Give them some time to settle in.
All of your fish would benefit from higher group numbers; they'll feel more secure with more of their own kind.
Supplement the Otto diet with some algae wafers and fresh blanched veggies (zucchini, cucumber, etc); toss in a piece at night after lights out and remove anything uneaten the next day. Corys also like to eat at night after lights are out and some shrimp pellets, algae or veggie wafers would be good for them.
Freeze-dried foods tend to cause bloat and other problems in fish. Soak it in some tank water first. Frozen foods are a better source than freeze-dried but either once should be offered as a treat once or twice a week, not as a staple food.
Try rotating the flakes with a higher quality food like Hikari or New Life Spectrum community fish pellets.
Also watch the betta; he may not like the other tank mates or they might go after his fins too, especially rainbows who tend to swim toward the top like the betta.