How the Utility/Heal spells workFollow

Ok so I know how weapons are incorporated into the skill system ( 1hMain=3 slots Offhand=2 slots 2hand=5 slots )
But the other 5 slots is what I am wondering for example i was looking up necromancer skills and I picked:
Frozen Grasp
Bone Fiend
Blood is Power
Summon Blood Fiend
Ghost armour

Now these are seperated into diffrent groups (http://wiki.guildwars2.com/wiki/List_of_necromancer_skills)
If you click on the link it will show you how they differ from one another
So basically I was wondering if you have too select 5 skills from the same "section" or can you mix them up like I did above^ ( these are the utility/heal spells it says btw )

Thank you if you decide to help :D