What small insects do ants eat?

Fire ants will eat other ants. Army ants will also eat other ants and actually seem to specialize in raiding the broods of other ant colonies.

In fact fire ants will eat almost any kind of insect they catch.

Ants (Formicidae) are a very broad genus. Asking what do ants eat is like asking what do people eat. It depends on which ants.