Well, I'll get the ball rolling by saying in my experience the 'right' eyepiece will vary from person to person, it's a personal preference thing.
But since you've helpfully given the aperture and focal length I'll add this advice:
The max magnification your 6" can theorectially provide is 300x (ie 2 x 150mm) So to work out what range of eyepieces you might usefully use I do up a list like this -
25mm = 60x
15mm = 100x
12mm = 125x
9mm = 166x
6mm = 250x
Also remembering that on an average night the atmosphere doesn't support magnifications above 200x (which would be usinga 7.5mm ep in your case.) I recommend three eps, one in a the low range, your 25mm, one in the middle around 100x and one at the higher end for the planets around 200x.
To answer your questions:
wide field or narrow field ep? - depends on who you talk to!
Personally I prefer a wide field for planets, but others may tell you to get rid of all that surrounding distraction and go for narrow field. And they may eve start jabbering on about exit pupil...
There are different types for different scopes but you'll find that the modern day eyepieces will all work well in different scopes as they are designed for the relatively short focal lengths of today. If I had a really old scope with a long long focal length that going back to a Huygens or Ortho may be worth it.
To sum up - you really just have to try different eps in your scope to find the one you like. If you join a local club or attend a star party this is not so hard to do - just put the word out you'd like to try some different eps in your scope and several people should pop along to show you their favourite ones. Otherwise you just have to buy one and hope it works. And as the americans say, "Your milage may vary."