Neons put out such a small load that you can keep a herd of them in there. Depends on filtration. You can mix them with live bearers like guppies, mollies, platys or zebra danios, gold danios, rasboras, cory cats, otos, shrimp, etc. For bigger fish it depends on how big your thinking. If it can fit in their mouth, it's food.
Sand is fine as long as you'r enot using an undergravel filter. The easiest plants are the java family (that includes both ferns and moss), anubias, water sprite, wisteria, and swords. These are ones I have the best luck with. The javas and the anubias are low tech, low light less demanding plants.
I keep ours at 80 degrees. They color up best that way. As for what they eat, they'll eat anything they can fit in their mouths. Ours have a varitiy in their diet: brine shrimp (live or frozen, whichever is handy), flake, freeze dried daphnia, freeze dried bloodworms, frozen bloodworms, veggie crisps by tetra brand.