Question: Is Jonnie correct in his assumption that there are other Psychlo colonies out there?
Answer: If there were other colonies out there, they wouldn't love as much as their home planet just like earth. Besides even if there was, they've been mostly defenseless without their fleet on the homeworld and will already be on the verge of extinction.
Question: Was it ever revealed how the Psychlos invaded and conquered the Earth?
Chosen answer: They teleported a gas drone into Earth's atmosphere. The drone flew around the entire world and dropped poison gas which killed Earth's population.
Answer: I believe that send an entire fleet of their mini-ships to earth by teleporting them (catching them off guard) and after settling on earth in the aftermath built a teleported on earth to survey the planet of its resources (unfortunately they couldn't find the gold which extremely valuable to them which was in Fort Knox, vaults and the Federal Reserve) and find an intelligent species to enslave to mine it.
Chosen answer: Between the book and the hideous box office take, no he is not correct. In the book, Psychlo got turned into a sun and all the other colonies tried to teleport their ore shipments home and were wiped out by the resulting explosion.
Grumpy Scot