As Zesty and slambros brought up, I believe Echo Fighters is likely just a re-branding of the term clones to hopefully move away from the stigma, though we'll have to see how well that works (which I sadly don't believe that it will.)
And as FrostShock posted, I also hope Chrom manages to get in as an Echo Fighter. It feels like it'd be a move in very Sakurai fashion to reveal him later with something along the lines of "Chrom Gets His Chance!". I mean he did just do the whole "Ridley hits the BIG time!"
As for why some clones seem to be Echo Fighters while others aren't I think it's just based on how different the characters are from the ones they're based on.
Going off of what's been shown, and assuming the rest is the same for now, we have this:
- Electroshock Arm instead of Upper Dash, but they're still basically the same move anyway; the weapon difference is more of an aesthetic change than a functional one. (Well sort of, stat differences, you get the point.)
- No tipper
Both - Minor stat differerences
On the other hand;
- Has a different Down Special; Dr. Tornado instead of Mario's FLUDD.
- Has MegaVitamins instead of Fireballs. And while yes they serve a similar purpose as a projectile they behave differently, whereas DP's Electroshock Arm does not. (It's differences being in KB angle and damage.)
- He has a new Down Air; it's in the video on his fighter page in case you haven't seen it yet.
- Stat differences.
- Retains a multi-hit jab.
- Fire Arrows in place of Link's normal (twin arrows now?)
- Stat and size differences.
- Retains his self damage mechanic
- Different Smash Attacks
- Stat and size differences
As for Daisy, she seems to share all the same moves as Peach as with DP and Lucina with Pit and Marth respectively. My guess is that she'll likely lack the ability to float and will have the usual stat differences such as being a little stronger or faster to make up for it.
With how minimal the differences between Echo Fighters and their base counterparts are, I actually doubt we'll be getting that many, but we'll just have to wait and see. They are pretty much just clones as we've always known them, at least based on what info we have so far.