PRO account: can I use themes from elegantthemes.com ?

sounds like a question for a WordPress.com forum:

http://en.forums.wordpress.com/
or
http://en.support.wordpress.com/contact/

if you have an account with WordPress.com, you need to ask at the suggested forum.

this forum here is for WordPress.org related questions – the difference http://en.support.wordpress.com/com-vs-org/

if you have a self-hosted site and are using wordpress.org software, you can use whatever themes you like.