I agree with GaGrown: if you can keep temperatures down, then you can add seedlings not too long after germination. The biggest problem with putting seedlings under an HID (namely HPS) is that the stems have a tendency to stretch towards the light, that is unless the light is very close to the seedlings. If you have an air-cooled hood, this is much easier to accomplish, and works like a charm...but if you don't, you'll risk possibly frying the seedlings by keeping them too close to the light. Do what works best for your plants. If they start showing signs of heat stress/stretching/etc., soon after being placed under the HID, then you'll probably want to think about keeping them going under less intense CFL's, or whatever you're using to grow your seedlings for a little bit longer.