Once again, I dislike the idea of the ARMS characters being a Hero/Koopalings situation since it goes against their individuality. Why would they do that when Daisy and Dark Samus are separate characters? It works for Hero because the Heroes generally have the same abilities and it works for the Koopalings because the Jr. Clown Car is 90% of the moveset so them having different models/designs/abilities isn't as jarring.
Dark Samus and Daisy were added over 10 years well after their base character's debut. Neither were a straight-out costume at any point either. They aren't an alt situation ignored. They were never real alts to begin with. Only Dark Pit was a straight-out alt to begin with, and then separated at some point(after he properly existed).
The situation here is closer to Castlevania, but we aren't getting any bonus characters, so Echoes won't happen. They are a Ness situation in how they work, combining abilities to make one character. That's already going to happen with ARMS, cause the ARMS are what matters as the core representation in Smash. The character abilities aren't all that unique in every case. Anybody can easily kick, do a comeback mechanic, jump multiple times, or use the Dragon ARM. Not everybody can teleport or float either, or use someone as a springboard. However, many moves can be inspired by others. Spring Man can use his hands to give himself a boost as a reference to Barq & Byte's springboard ability. Not all moves are created equal overall. In Smash, you have well over 20 moves. No individual has 20 moves in ARMS itself. There's less due to how the controls work. Small tidbits/references to others is already inevitable. This doesn't mean using the same animations either, as that's where the real individuality lies. Which makes sense why an alt situation is not likely. That doesn't mean a frankenstein moveset is unlikely however. It's already going to happen due to the ARMS being the most important thing for whoever is chosen, but a lot of the abilities are actually generic anyway for others to reference.
Alts =/= using the abilities of others without using the exact animations(these are very different arguments and why people still expect outright an ARMS character will use some moves of another one regardless of whether they also have a direct alt or not). While Ness' lack of animations were brought up, this isn't a very good point in itself either. PK Thunder is designed where it doesn't target a particular character and can hit different enemies without a particular pattern to it. In Smash, this is how it works, not being in a straight line and goes all over the place. PK Fire can hit some enemies far more damage than others. This is represented in Smash by it staying on the field for a while by doing lingering damage, to capitalize on a burning-like effect. You can retool moves to easily fit the settings regardless of animations. While Lucario is clearly based upon his anime abilities and fighting style, it doesn't use the exact same animations all the time, taking liberties. This is to be expected to make it work. As I noted before, Spring Man can easily kick. He won't kick identically to Min Min because they have different overall animations, but it'd still work practically in the same way anyway, since it's an easy to do move. If you want a good example of this, look at Isabelle, someone who doesn't actually have a canonical ability to do a lot of stuff Villager does. Pocket and the Party Popper she shares properly with Villager. She's never once used a Tool, but she has handed certain tools like Party Poppers and Glow Wands(they fall under Tool gameplay within the series) and is easy to believe she can use generic items like that. Other animal villagers literally use that kind of thing. That's what people mean by generic gameplay being interchangeable enough to make for an easy Ness situation.
...Let's just say it's not really easy to explain every detail, but a Ness situation beyond the ARMS is very likely due to certain moves being generic enough to work. Ness had more canonical abilities than any particular cast member of ARMS too, but a lot of those moves weren't plausible within Smash either, nor make for a coherent moveset. The importance is coherency in making it work, not how accurate it is. That's why Ganondorf never fully used anything but his non-canon broadsword as a proper part of his individual moveset(the custom specials and taunt work without disrupting his moveset flow for TP Ganondorf, but they don't work for OOT Ganondorf because his moveset is brute force. A sword designed more around fencing doesn't flow well). It's why Pikachu only resembles its anime appearance while not working similar to the games much at all. Skull Bash is a great example of having a very clear animation being ignored so it makes more sense in Smash. There's no rearing the head back. It's just sitting still in a stance and then flying off. The anime also did it differently from Smash and the games. This is the kind of animation liberties being talked about, and happens with most of the cast as is. Or abilities that aren't accurate(Monado Arts loosely resembles Xenoblade Chronicles' system but makes sense for a Smash moveset). Etc.