I don't use any additives, except for whatever the stations use to make winter blend fuel in the cold months if that counts. With the amount of miles I drive, oil filters are changed every 6 months and fuel filters once a year. Draining and flushing other fluids will be per the maintenance schedule in the manual. Beyond that there's not much too it except for the normal stuff like replacing tires and batteries and such.
I'm not sure what you're getting at with your question about spark plugs and glow plugs. Your Cummins has neither, though - it uses a grid heater instead of glow plugs.
Your injectors should last a long time. Really the word so far on the current 6.7 is that Cummins got it pretty well dialed in, and the reworked emissions with the addition of DEF really helped fix some of the issues people were having with the previous 6.7 - better mileage, fewer regens, no EGR clogging, etc. And it still uses the old style CP3 injection pump which is both less prone to failure than the CP4 and when it does fail you simply swap in a new pump and you're done.
Congrats on the truck. You'll love the way it drives and pulls.