Bio missile is an awesome idea, and could be a way to do the ship and ground unit takeovers. Perhaps partial takeovers for balance sake? For example, fire missile at ship with hole in armor and the game reduces the defense strength of the ship against boarders and creates a small boarding party on the ship. Taking over a ship this way should be very tricky, though. Remember, ships have airtight bulkheads that could very much limit the extent of the spread of the biological agent.
Yes, your homeworld would be a world on which you assimilated the natural inhabitants, but that is the point. All of the natural inhabitants would already have been assimilated, otherwise another race would have to be created on the same planet.
If they did land on earth with no military, then yes. I do see the invading population wiped out by defending ground troops in relatively short order. However, that is not to say that it would not be POSSIBLE to grow faster than the troops can kill you, especially if the planet is somewhat undefended. In the end, you would still have to land ground troops to clear out the enemy's military, but if you steal structures as you steal population, then things could get interesting.
Redshirt, you are probably right with regards to the racial genetic research. Forcing the race to undergo genetic mutation to be able to take new hosts seems as if would completely neuter the primary capability of the race, however, especially considering their already weak research. A new tech tree being created as soon as a 'sample' of a race is obtained (lifepods, espionage team, invasion, etc) that governs conversion speed of that race sounds like a good idea. It could shore up the rather anemic genetics research subsection. I would leave a base conversion speed, however, just for balance. And for the sake of simplicity, machine races should not be immune. We can say they use their covert skills to reprogram the computers or something.
