I think Fei adapted surprisingly well to the past, I know this isn't shown in the anime because it's not relevant but I would LOVE to see situations involving Fei failing to fit in Tenma's present, for example my little HC is that he speaks differently than the rest of the Raimon, futuristic terms and all that. But everyone is okay with that, I also HC that he loves history (this is like half canon, he seems to enjoy timetraveling) and since he's a lil history nerd that's why he managed to blend in Tenma's present.
Fei surprised with the texture of the (for him old) Raimon team uniform