cheap-o colony ships that are just colony modules and cargo hulls, will work, but i'd recommend putting ion drives on them as soon as you can. they're cheap enough to justify putting them on.
The Korx do start off with Ion Drive, I didn't see that. 85bc speed-1 colony ships vs. 105bc speed-2 colony ships, are 20% cheaper. So the tradeoff, a 20% faster build time (which means 20% more ships) vs. a 50% faster speed (with +1 speed bonus, it'll be 2 speed vs. 3 speed). Yeah, that probably is worth it to slap on an Ion Drive. When you don't have the speed bonus it's a no-brainer--you put an engine on.
But then you research Impulse. Now it's only a 33% speed improvement (speed 3 vs. speed 4) to slap on an Ion Drive. Unless you've got a long way to travel I wouldn't bother. Especially if that's the difference between putting out a colony ship every turn vs. every other turn.
Hmmm...if Korx start with Ion Drive, is +1 speed really helping them that much?