Is this a good set up or too much for 20 gal high?

Overstocked. And Oto's need groups of 5+. I would cut down to 8 Cardinal Tetra's and get 5 Oto's. You could do the Assassin snails and the shrimps too. Shrimplets will be eaten though. In a planted tank with regular water changes you could do that. And watch that that 50 gallon filter doesn't produce too much of a current. And 76 is fine for Cardinal tetra's but I would bump it up to 78. The plants aren't going to die because of two degrees but the fish will be happier for it. 2 weeks without the fish in the tank is fine. The Ich should be gone. I'f you're that worried remove the invertebrates and plants and dip the plants in a salt solution and dose the tank with some aquarium salt then add everything back in and do a 50% water change to dilute the salt before adding other fish. But other than that you should be okay to add more fish at this point in time slowly to the tank as your tank is running on a low bioload at the moment so you'll need to increase it slowly.