Impatiens are, more or less, shade plants. They will remain compact even in quite low light. The Zinnias and Cosmos are the opposite, they get leggy even in full sun in England. Even perfectly healthy plants are tall. The answer, as always, is more light. And more of the light you already have concentrated onto the plants. "Cooler", higher kelvin temperature fluorescents should theoretically produce more compact plants but I haven't seen a difference.

Thanks for the heads-up on natural growth habits. I already have the seedlings growing under 3 2-foot fluorescent fixtures, so I guess concentration rather than amount is the main issue here (I didn't know which colour to use so I got all 3!). Perhaps I started them a little too far away from the lights :)