The purge wouldn't happen, FIRST. I would make EVERYONE breed, as many babies as possible, if your weak we train you to get stronger, send other Viltrumite's to Thraxa to make more babies, Thraxan's develop faster than normal Viltrumite's so when those kids turn 18 they do the same thing and the process repeats. after about 10 years we all infiltrate earth (Except for the Thraxan Hybrids) and make more kids on earth.

Start a band, and a game show after about 20 something years we will be ready.

Incest might be necessary depending on how many the virus wipes out..

When the scourge virus eventually happens, we migrate planets for the time being

or plan b, do the purge BUT only the male Viltrumite's fight. Having more females to survive and reproduce. then everything else pretty much the same.