It's interesting that you bring up ableism and internalised ableism as a parallel between Shadow and Tails because I have a headcanon that since we don't really know how the rest of the ARK took to Shadow compared to Maria, Gerald and the Commander, while he was at least accepted by his adoptive sister, the other inhabitants were afraid of him and/or didn't treat him like an actual person due to his origins, similar to how Tails was ostracized for his polymelia
Oh SO many thoughts on this. I really want to write I proper in depth post at some point but I want to answer this NOW so just uhhh very roughly for now:
SHADOW. His particular brand of internalised ableism does stem from how he was treated on the Ark. Before I say anymore I'm gonna quickly note i still haven't finished playing sa2 or watched the cutscenes of shth, so take this with a grain of salt. The scientists on the Ark were very ableist towards Shadow, but not intentionally or in a noticeable way. He was Project Shadow, the ultimate lifeform, and so when he acted more like a person, flaws and all, and not like a Project, and especially when he showed symptoms of autism, he was reprimanded, told that there wasn't any excuse that he should be better.
I think about this a lot, I could say and explain more but I won't yet. Yes Gerald and Maria loved him, but this was the 50s and also people who love you can hurt you. Gerald hurt him sometimes, careless words and actions that cut deep into Shadow's soul. Maria's also autistic, and she never hurt Shadow, she was nothing but loving and supportive of Shadow. But Shadow had to watch Maria receive the support that he was denied because he was supposed to be Better Than That. Maria did her best to extend that same support to him, to make everyone else treat him the same, but there was only so much she could do.
Most of Shadow's internalised ableism comes from that way of thinking, he's supposed to be Above that. He's the ultimate lifeform!! He shouldn't be acting or feeling like this, it's childish and stupid!!
TAILS had similar when he was younger! the dehumanisation, the being told you should be better than that etc etc. I'm getting really tired I'm gonna have to wrap this up soon. And he put that thinking on himself - for a start it was a survival/coping mechanism, he was abandoned and left to fend for himself, so he told himself that he couldn't afford to be childish, he was above it all. He didn't have anyone to lean on for support, and he couldn't provide it for himself, so he convinced himself he didn't need it.
Then he met Sonic and suddenly he HAD that support. long story short, it took a lot of time and effort to deal with that particular brand of internalised ableism, and he still struggles with it sometimes, but Tails found his support system a lot younger than what Shadow did, and has more time to start to heal. Shadow has only just started that journey. This is making it sound like Tails is 100% fine now, I promise I'm not saying that I'm just really simplifying things
AND SO Shadow sees himself in Tails, a lot. Especially autism wise. He sees tails exhibit the same symptoms as him, ones that Tails has grown comfortable with and accepted as himself, but one's that Shadow still believes he should be above, that he should be BETTER. And Shadow projects on to Tails, hard, and ends up taking it out on Tails, and so all that internalised ableism just becomes regular ableism. Shadow genuinely doesn't mean to be an asshole, in his mind he's trying help Tails be Better, but like. Still ableism, dude
I hope all this, especially the last part, makes sense. I'm so sleepy I'm about to drop off at any moment. I want to talk more about how Shadow projects his internalised ableism onto Tails in more detail but later
