I got to say that I love my Dwarf Cory's, I also have a pH of 7.6 but the one problem is that they are very active swimmers so they might get stressed in a smaller space. When they are happy they are swimming up and down the tank walls and then munching on the algae that's on the plants and playing in the sand. You can still try them but just be warned is all.
Also, they do well in sand or a smaller smooth gravel because in regular gravel their barbel's can get hurt and that's no fun for the little guys.
You can get more than 2 Amano, you could actually have about 10 in there and it would be great, Shrimp have very small bio-loads so you don't have to worry about them pooping all over the place like snails lol.
But a Nerite is a great choice in snail! It has the least amount of bio-load and cannot reproduce in freshwater! So I think 1 will do you great, remember you also have to acclimate them the way you'd acclimate a fish. Shrimp are very delicate and sensative so I would recommend Drip-Acclimation for them :)
And 1 Betta, that sounds great to me! The Amano's will stick up for themselves generally if the Betta decides he wants a snack, however still be warned, your Betta might still want a midnight snack! So plenty of hiding places for the shrimp is good, lots of pants for them to munch about and hide.
Over all looks great :D
RJ (H. Lifalili), Acara (EB)
Ghost, Trip (Angels)
Leopards: 2:3 Hawkeye, Cullen, Luna, Astrael, Dorian