I dont think they plan on just taking brawls models and retrofitting them, every character has a finite amount of detail that will have to be done, and besides other m's ridley easily has the most muscular frame which might work better than his super thin other variants.
She had flat shoulders in zero mission when using her power suit, when it switched to varia and gravity it had the shoulders. You can tell the difference between the super metroid and zero mission designs based on the panel lining and lights of the suit, they definitely used the zero mission one, it makes sense since they were going with the version that brought us zero suit samus anyway.
This is the Varia Suit, taken from the cutscene when she escapes from Zebes after Mother Brain's defeat. She also have the small flat shoulders in-game with the Varia Suit. It is but a more orange shade of her Power Suit in that game. Zero Mission and Fusion remain the only games where she doesn't get the big shoulders when acquiring the Varia Suit.
However, in Super Metroid she does lack the "eyes" on the chest piece of the suit that she has in Zero Mission on her Gravity Suit, and also the lights on her legs, so in reality Samus doesn't really appear in any of her games like she does in Brawl, closest would be Metroid Prime, but given that her Varia Suit from Prime has its own trophy it's most likely not based off of that. So it's more like her usual Super Metroid color scheme attached to the Zero Mission kind of model.
And the muscular form of the Other M Ridley model is exactly what might make it harder to animate and fit. The thin model has much more room to move in, where as when scaled down the Other M model would either need to be airborne or be in a very spread position before he would look right. The thin version of him would have a lot more freedom in that regard. Ridley's head in Other M is also smaller than in any of his other appearances, so they'd have to scale his proportions pretty heavily to make him fit, which is something they rarely do with characters. Using the Brawl model as a place to work from would be much more efficient and much easier than having to go through everything with the Other M model. Yes, it'd still need a change in proportions too, but it isn't as hard to do as it would be with the Other M model, again, because of the muscles. So unless Sakurai is hell bent on getting a model that's sure to get a lukewarm response then I don't see it happening
Yes, every character model is touched upon, some are mostly just texture edits, like with Mario and Sonic, others require a little more to fit in, like Bowser, but they still use the models from either previous Smash Bros games, or other games from the respective franchises, like how they used the Twilight Princess models for the LoZ characters. Some models are created from scratch, like Pit, but I think it's evident that if they can use a model from another game, they're going to do it.
Taking the Brawl models and updating them certainly isn't out of question, they did use some of the Melee models as a basis for the Brawl models, which can be seen with C. Falcon, Samus and Peach. I'd be surprised if they didn't do that with any of the characters this time around.
I think Other M will be represented with stages, music and trophies, exactly like Prime in Brawl. I find it hard to believe that it'll be represented on the characters too though. One of the reasons I think Prime probably wasn't represented so much was its subpar sales and reception in Japan, Other M is exactly in the same boat, except that it didn't even sell well or get good criticism outside of Japan either.