Neither Gardevoir nor Gothitelle run normally, both float in a very similar fashion, albeit Gardevoir's being more energetic, but it could be tweaked for either or both in Smash, though it's not like Ganondorf ran like Capt. Falcon.
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All that would really need to be done is altering the dress effects. For other movements, Dark Samus already shows us how different animations can be between the Original and the Echo.
You're also basing how they jump off of the anime, presumably, which Smash doesn't have to follow 1:1. Gothitelle's 'dress' is in sections, it could believably bend similarly to an accordion.
Now I don't believe the leak is true, regardless of how much I want Gardevoir in the game, but there are various ways to make Gothitelle work as an Echo Fighter of Gardevoir..Sakurai is a very creative guy to begin with and they do have similar animations in the games themselves.
Not to mention we're all saying Dixie could be an Echo for Diddy and you know a lot of animations will have to change there.