Yes. Dima (Дима) or Dimka (Димка) are way more common diminutives for Dmitri these days. As for Nikolai you can use Kol'ka (Колька) together (interchangeably) with Kolya, the former sounds a bit more informal and can also be used while arguing to express impatience, irritation, annoyance or something like that.
Calandra, you're welcome. Feel free to ask any questions.