Among Dixie, Dark Samus, and Shadow, the only one that has less chance of an Echo is Dixie, but that's solely due to animating her hair. This takes a lot more effort than what is expected of an Echo. Not using it for moves, but the fact it exists. This means they have to take it into account for the entire moveset. This may move her into clone or semi-clone status by default. The other two have a near identical build or same build and don't have any animation worries. Albeit, Shadow would look better with karate chops and such, but he can easily do all of Sonic's moves anyway. Dark Samus doesn't have access to Samus' arsenal entirely, but wouldn't look weird doing the same moves due to wearing the same suit. Some various shades of phazon added would be enough to make him look right. Not unlike how Ganondorf's magic is animated.
Dixie is capable of doing all of Diddy's moveset so far that we've seen. It's just the animation that makes me wonder if she'd be a regular clone instead, with mostly the same moveset, but smaller changes(basically a Melee clone), and perhaps one unique special. This type of animation thing is the reason why Daisy can't be in her Sports Outfit, due to the fact the dress is an attack. But also because it would require way more work than an Echo would.