Neons will be OK with any small fish that need their water contitions to be slightly acid pH6.8 and over 78F. That includes most tetras, most rasboras, most danios, most barbs most dwarf cichlids most gouramies, most corydoras.
Yes, guppies can live with neon tetras. They in fact make good aquarium neighbors as guppies are top feeders, neons are middle feeders, and the temperature requirements are the same.
Yes,...
Many species of fish will mix with neon tetras (Paracheirodon innesi) they include all of the smaller Barbs, Tetras, Rasboras, Corydoras and Dwarf cichlids (Amazonian). The big things to remember are...