I was thinking of a race along these lines, actually. Something that can convert enemy population to your population. The problem I foresee with these guys is how do you start? Your home planet would be unable to grow in population as all of it is you. I would suggest a vastly reduced rate of growth when not converting enemies, but not NO growth. Also, the taking over enemy ground units is a bit unnecessarily complex. Instead, how about they can only take over civilians? But the process is automatic as soon as they have a positive population on the same planet as another species. Also, their populations cannot be conquered, only exterminated, something which is done automatically by unopposed ground forces. Capturing ships is cool, but would work the same way it does currently (except perhaps better and you keep some of the crew's skill). Instead of a bonus to disassembly, perhaps a bonus to espionage research theft instead? Perhaps something like this:
Perks:
Bonus to mining
Bonus to industry
Any population on the same planet as an enemy population acts as a skill 100 espionage team.
Any population on the same planet as an enemy population slowly siphons off the enemy's population, converting it to your race, at a normal growth rate.
Ability to infect and capture enemy officers.
No morale penalties.
Penalties:
Reproduction without conversion (no enemy race) is slowed.
Reduced RP generation. (not greatly reduced, just 100 base or so)
Undefended populations are rapidly exterminated by enemy ground forces.
Once encountered, tech trees will open up for enemies slowing your conversion rates.
Initial diplomatic position is hostile.