Taking a core province from Netherlands (or any other significant European country) is going to cause trouble. You MIGHT have gotten away with taking a colonial concession (Jambi or Java, for example), which could be fairly beneficial down the road a few decades when it comes time to colonize and now you've got a port within range.
Since you can only ally with 2 GPs (and most of them will already have 2 allies by the first or second day of the game), it's a case of taking advantage of any breaks in current alliances to pick up an ally worth having. Unfortunately, the UK is a fairly untrustworthy ally in far too many games, where when push comes to shove, they say "sorry, you're on your own". I quickly noticed that relations really doesn't do much in this game, so +200 relations really isn't all that useful, compared to in several other Paradox games.