Can Premium Plan users use specialized Dictionaries?

No they can't.

Only Yaraku Dictionary is available as a default dictionary for the for the Personal Plan (both Free and Premium) and Company Plan (regular) users.

For the Translation Managers Yaraku Dictionary and Genius Dictionary are available by default.

Company Plan users can sign up for other specialize Dictionaries as a paid option.

Please refer for the more information under Integrations>Dictionaries.