It occurred to me to check how the DLC section of the character list is structured on the site. I figured it might have been hard coded to fit the 6 DLC characters exactly (in which case, echoes might not be likely), but nope, the site is actually set up to, in theory, add an infinite amount of DLC fighters without any issue.
They're literally just <li> elements and the site scales as needed. As seen here, I just added 2 more <li> elements and the site adjusted as needed. It scales and adjusts the grid as needed when resizing the browser, no matter the amount.
Not that I think they specifically set the site up to add echoes (responsive design is a good thing to do), but, in theory, they didn't design the website to
not accommodate additional fighters if needed.