Your idea of schooling is not really attainable in a 60 gallon tank. Chromis are not actually schooling fish. They shoal. Which means they group together in the presence of a threat. They also get bigger than you think. I originally came into the hobby wanting a group of them also, but after tons and tons of research, and reading people's experience with them, I abandoned the idea. The fact that they are so commonly diseased was the icing on the cake.
You can get 15-20 snails easy, depending on what kinds you get. Turbos and trochus and massive eaters. Ceriths are much smaller, dwarf ceriths even tinier. But I have about a dozen turbos, I had about 10 trochus until my hermits killed all but 1 of them. A few dozen dwarf ceriths, a handful of nessarius in the sandbed. And about a dozen hermits.
Hermits in my experience are not terribly effective cleaners at all. And I regret getting any blue leg hermits because they have killed so many of my big helpful snails.
Don't worry about the damsel/chromis right now. Get the other fish you want. Get the angel last. This will take many months. Who knows how you will feel by that time and how the tank community will form. I changed my stock list 487 times in the 16 months I spent deciding on fish.