Chrom’s an echo fighter, but no rebuttal for Rex, though it’s weird he’d include a Mii costume of him in the first fighter pass if he was planning on adding him later. Unless he wasn’t
It's not weird. The costumes, especially first party(third party might be more awkward due to how licensing can go many ways), are fun things and Sakurai doesn't follow these kind of fan rules. Base game doesn't apply to all DLC, especially a second set.
Rex wasn't planned for playable in Pass 1, as Nintendo didn't suggest the character to him. Thus, he got a MIi costume because he was notable and someone he considered for base.
Pass 2 was greenlit later, and Pyra and Mythra are the core fighters anyway. Rex was just like Pokemon Trainer. Still a part of it, but didn't have his own numbers.
Chrom is just more proof Sakurai doesn't actually care if they have another role. It's just that the first DLC is going to be planned around the overall base game's finished character roles, so rarely a character will be in a special role and playable. It's not some guideline, it's "we already weren't going to have them playable" and the first set of DLC is easy to look at as continuing the base game. Keep in mind a Pass 2 wasn't originally going to happen.
This doesn't mean a MIi Costume or AT will be upgraded. It just means they are hardly as important as people think they are. We already pretty much know MIi Costumes aren't supposed to mean anything on their own. Nintendo, being the ones choosing the DLC, only care about business. They are just going to suggest interesting characters, often tied to business decisions and/or fan requests. Not cause "oh, hey, we see this guy has a role." That's really not where their thoughts would lie. It'd actually be really bad business to make some asinine rule like that. Now, what is realistic is Sakurai would've denied characters he already has a role for just for the First Pass alone. It probably happened, Sakurai did note nobody was denied for moveset reasons, but anybody else could've been denied in the First Pass for many reasons.