Leopard (well, many types) danios: generally go by size and girth--females are generally bigger and "fatter" than males
Neon tetras: much the same as above
Sucking loach: can you be more specific as to which type you are referring to?
Common plec: from what little I have read (which was pretty much books that would now be considered outdated) it's pretty much impossible to tell the gender. *shrugs*
Maybe someone else might be able to help you out more than I...
A sucking loach is probably a Chinese algae eater (Gyrinocheilus aymonieri). According to Baensch, sexually mature individuals have thorns round their mouths, and males have more than females.
