Subclassing sounds complicated. Do I have to do it or can I have my characters remain pure?
You don't "have" to use it but ... there's really no reason not to. It's pretty simple. Select a class as your subclass and you get access to their skill-tree (sans their Class Passive, the Proficiency line) with half of the points possible (e.g., a skill that goes to 8 can only take 4 points if it's subbed) as well as a free +5 Skill Points to use however you want.
The difficulty of subclassing is just figuring out what skills are really, really good and benefit a your main class really well. For instance, an Elemental Landsknecht benefits greatly from subclassing Runemaster for the passive skills that give +Elemental Damage and +Damage when hitting a weakness.
Also for my class setup I'm thinking:
Bushi, Fort, Dancer: front line
Runemaster + Sniper/Nightseeker: back line
thoughts?
Just to note, you don't start the game with access to Bushi so it'll be awhile before you have one.
Besides that, you'll most likely want a Sniper, not a Nightseeker, for the backrow. Nightseeker is a melee class. The only thing they can do from the back would be their throws and that's not really that good of a use for them, honestly.