The orginal question was regarding having only Categories displayed without sub categories displayed in the hierarchy.
If you want a WordPress Plugin to only be used on one particular page then you will need to create a custom template page and this will require a fair amount of coding knowledge and coding modification to get the plugin to only display on your custom template page. Otherwise your category widget plugin will display on all of your WordPress pages.
I also found another WordPress plugin that may interest you if the Ajax Category Dropdown plugin does not do exactly what you want it to do. That WordPress plugin is called >>> Multiple Category Selection Widget. Good Luck!
PS Your first priorty should be Search Engine Ranking, then the visual appearance of your website. Why? Because if your website does not rank very high in the search engines then no one will ever see it anyway.