• is it required to warm up your diesel before driving?

Solutions for this would be, remote starter. Or the BD Idle kit, or an exhaust brake to aid in warming up quicker.
BD Power - Product: Dodge High-Idle Kits

BD Power - Product: Exhaust Brakes

• is better to leave running, or just shut it off if only going to be 5 minutes?

The BD cool down timer will shut the truck off at a set temp or time. If anyone tries to steel the truck the second they touch the brake without the key the engine shuts off.

BD Power - Product: Cool Down Timer 2

• can you fill the fuel tank with it running?
Yes but people get goofy when they see it.

• diesel engines go longer intervels before oil changes?
Always check the owners manual

• What temperture does diesel gell?
Using a good fuel additive helps reduce this issue, as well, some run fuel heaters.