Further reading of interest:
According to the biography printed in the Transformers Collectors Club magazine, Slag and Sludge were part of Grimlock's band of Decepticon hunters on board the ship Graviton . When Grimlock left Ultra Magnus in charge to join Optimus Prime on Earth, Slag and Sludge departed the Graviton , not wanting to be under the command of Magnus. The two Dinobots went off on their own, and happened across Nightbeat. Nightbeat offered them a job as muscle, which they took because they figured Nightbeat has a nose for finding trouble, which they enjoy.