I agree that it would have made all kinds of sense for Noldorish as well as Sindarish types to get along all of the time. Why not? Too bad that it didn't happen often enough. Anyways, yes, Eol didn't like the
Noldor types. He was thinking stereotypically. Yes, that is a bit
evil, but the stereotype that he had pinned to them was rooted in sensible caution. At least he wasn't the sort to kill large groups of people who annoyed him.
Also, how did Aredhel not turn Maeglin against his not-so-adept-at-social-rituals father? She might have had great reasons for doing something evil, but she still intentionally manipulated the kid. Maybe even thinking mostly of the kid's welfare. I don't know.
Tolkien doesn't go into much detail on her intentions, from what I have read. I gots to point out that different
elves have different tastes.
You write that life in
Gondolin (where, even if you aren't someone who's job requires him to work outside of the place and have to be plenty careful about secrecy, the Enemy might just catch you and mess with your brain, since he's always on the lookout for that hiding spot) is better than life at Eol's place. Certainly to the superly spoiled Aredhel. I'd need all kinds of parameters to decide on which place was somehow better.
Is living isolated not good for a child? I am not an expert on child rearing. Most children raised in especially isolated
places turn out to be evil? Hm. At this point, I wish that I was an amazing history expert, to point out the many popularly seen as great humans who grew up in mostly isolated spots. I see no evidence that the kid's old stomping grounds turned him evil. The only ones that I know that he hung out with were his parents, some servants, and some
Dwarves, who are always great examples, known for being untaintable by evil.