They start to get colour at around 2 weeks Robbo tho you might see a few spots appearing after a week or so.
Sarah, since this isn't the first time you've had live-bearers you'll be aware that the adult female will give birth every 4-5 weeks even without the continued presence of a male. They can save sperm packets to fertilise up to 6 batches of fry. So I think your policy of leaving them in the main tank is the best solution as only the fittest will survive and you won't be overrun. It also means a reasonably low increase in bio-load for the bacteria to accommodate.