Yes, puffers and loaches are not compatible with snails. Most african cichlids are not compatible with plants, but they are all way to big for a 5 gallon anyway.
A 5 gallon hex, guppies, white cloud mountain minnows are pretty good choices, as they stay small, and are somewhat more tolerant of poor water conditions, will not bother plants or snails. (Though I'm not really sure about the fancy-tail guppies, they've been bred so much for color, I don't know their hardiness.) Avoid Neon and Cardinal Tetras, unless you are an expert with them. Their size is right for that tank, but I find them to be very sensitive to water conditions, and they croak easy. With a 5 gallon, you will have to stay on top of water conditions big-time. The water parameters can go bad on you very quickly in a tank that size. Some plants are a great plan for a tank that small, plants will help stabilize water conditions, especcially some fast growing stem plants like Hygro, water sprite, or my favorite for small tanks, Hornwort.