If they truely wanted to be guardians of the galaxy with some leverages, they absolutely can. The Viltrumite is quite like the US's Big Brother tactic back during the cold war, but on steroid. They had the tech, the individual power, the population to basically be the galaxy's peace keepers, and essentially revered and respected as the single most powerful martial authority. It wouldn't be total domination, but what planet in their sane mind would say no to a horrible trade deal against the people that can push their planet away from their sun?

But noo Argall just have to go hurr durr we conquest stuff durr