How can Niko work so immensely within the Pegorino Family?

Couldn't the same be asked about Phil Bell and Dimitri Rascalov? But aybe they're not as close as Niko.

Niko isn't Italian so wouldn't he just be an outsider and not be involved in as much as was?

Honestly, I think that's one of those "Don't read too much into it" sort of things. You could say the same about Playboy X as Niko was definitely not a part of that clique. Or Elizabeta. The simple answer is that Niko is a hired gun and any corrupted individual could've possibly hired him. He and Roman needed the money. People with as much criminal influence as the Pegorinos definitely would use any hired gun they could get. Of course, Niko was still a man of principle so it's unlikely he would've worked for just anybody or continue working with them. That's why most of his employers end up dead for the most part. Lol.