Usually PT Express comes with Mbox or M-Audio/Akai interfaces. But yes, you can use other plugins:
http://avid.force.com/pkb/articles/en_US/FAQ/en436171
Q: Is Pro Tools Express compatible with third-party AAX and RTAS plug-ins?
A: Yes, Pro Tools Express supports AAX Native (32-bit), RTAS, and AudioSuite plug-ins from Avid and our development partners. You can find these in our Plug-in Finder.
Joel