Dixie
can
Dark Pit Staff is based on Light Arrow, so no.
Special as in Special Move ... B buttons. Not final Smash. So for that question, we do not know for certain at this time.
As for other Echoes, Dixie
can be an Echo of Diddy. From the presented videos, we do now know if he has his tail multi jab ... it may have been changed which can fit better for Dixie. Also, I do agree, a variation of DK's recovery could work for Dixie.
A list of other possible Echos in the future (DLC). Not a question of probably, but just possible in the sense of mechanics:
Mario Series
Mario & Dr. Mario: I personally don't see a need for an echo because of these two. It almost seems enough as is.
Luigi: One can argue since Mario, Dr. Mario and Luigi can be grouped due to their similarities. But for the sake of it, a Dr. Luigi could be used as an Echo of Luigi ... so both brothers have their PHD. The notable change would be to have the Down-B move converted into a FLUDD like mechanic. Instead of water, it sprays a colorful "medicine" spray based on the pills.
Peach: Got Daisey.
Bowser: Dry Bowser. Popular "character" because of the design. Also was an Alt. Skin for Bowser in Project M. The notable change is Stand B is changed to blue flames that do more damage, but diminishes faster.
Bowser Jr.: Got the Koopalings as Alt. Skins ... so not really needed.
Yoshi: It appears the "Crafty" Yoshi is an Alt Skin. Birdo is possible if the Standard B is changed to be a sucking air animation instead of a tongue. Up-B visually changes to Birdo shooting eggs out of mouth instead of Yoshi throwing them.
Wario: Can't think of anything, he is pretty unique.