Patrick and Peter's was originally a preschool friendship--they couldn't have been much more than 6 and 7 when the war broke out. Even if their personalities had suited at that stage, that's not going to stay the same till their teens! Whereas Patrick and Nick don't really get to know each other till they're 13 and 15, and they've already grown into the same general sort of person before they meet, which I don't think Peter has.

I don't think that Nicola has any romantic feelings for Patrick before he goes gooey over Ginty; it's only really by the time of Run Away Home that I get the impression that Nick sees herself as a girl rather than a 'N. Marlow, RN' whose chances have been spoiled by inconvenient femaleness. :)