Translation:People are clever.
Yes, quite often. It's mainly used for emphasis or when it is absolutely necessary to distinguish that something is plural.
General concepts do not require the plural suffix. Otherwise, it is not common to omit it after an animate noun unless its plurality is otherwise implied.
In "영리합니다" do you omit 'ㄹ' when pronouncing it? Which Makes it sound like "yeongihamnida" because i cant tell with Duolingo's example~