What would cause an custom element listing to just return everything?
It is likely just a typo in your template. Can you ensure that if your element type class name is of the format Custom_PluginElementType that your template code is then in the following format:

{% set elementType = 'Custom_Plugin' %}


Or if your element type class name is of the format CustomElementType that your template code is then in the following format:

{% set elementType = 'Custom' %}