Here are a couple of other ideas;
Parasites- (aka; The Vang-by Christopher Rowley, Genestealers from 40k, the Parasites from Invasion of the Body Snatchers)
The race is parasitical. They grow everything from organic material they assimilate from other races. In the Vang books, the Vang themselves are basically neural material, they take over a host and proceed to bioengineer it to suit their own uses. So, if a few of these get loose and start replicating, it can overwhelm a colony fairly quickly. The plus side, is their ship building takes a while. In the books, they were endangering the galaxy, and got stopped when another race used a weapon that caused their sun to go nova. Genestealers infect populations with their genetic material and co-opt their maternal/paternal instinct to protect the stealers. Lots of nasty fun to be had with these guys! Not as dangerous as Consumers, but harder to kill off. Left unchecked, very dangerous.
Consumers-
Basically, this is the pattern the Swarm fall into, but you could rev them up a bit my adding in ground troops and bonuses for the worlds/biomass they consume. The bigger the biomass, the larger they get, so they could be a potential game destroyer if they hit a very "green" system. Just to give the Invaders something to worry about!

These guys would be your Star Trek Borg, or 40K Tyranids. Very scary bad guys!
Raiders-
Because we still don't have Pirates in Aurora!

Star Vikings, Pirates, slave raiders, ect. A race that isn't necessarily out to conquer a planet, but the DO want you resources/tech/population. Not so much a game ender, but annoying as hell. Dangerous in the early game, more annoying later on, they just would add a random "WTF?" factor to the best laid plans of Presidents, Emperors, and Kings. Might make for some fun RP as well.
