I think it would be a good idea to have a different recruitment system for the underworld (at least for GC). I believe some units should be rather hard to find (droidekas, for example), so the amount you can recruit at any one time should be limited. I'm not speaking about a popcap for rare units, but rather have a random number of them available to buy.

For example:
You see that you can purchase 3 droidekas on a planet, because you've "found" somewhere to buy them from.
Some time later, you could probably have some more available for purchase.

I just don't think it would be a very good idea to have every unit available for the underworld in large numbers even if they have the money for them.
This way, some specialised and rarer units could be powerful without imbalancing the game and you wouldn't have an army of 20 uber units late in the game.

yea but i reakon that the underworld should be the weakest in total strength as they should only be able to build small ships coz some ones goin to notice if you build a big ship over an imperial world

i reckon that its a good idea... it would ensure that people have to play the Underworld differently to how they play the Empire and Rebels... it would help make the game more interesting...