what i did was keep mine on a window sill for a week or so unltill they have a few good sized leaves, and then put them under the light, worked really well.

4-8 plants under that light will be fine, ive got a 400w with six plants under it.

the thing with your set-up as it is....is that a 400w with those small pots will dry out faster. saying that, overwatering is probably your biggest danger in a small pot. as long as the pots have good drainage you should be ok.

the top layer of soil will dry out quite quick in those pots and that light. but that dosent mean they need to be flooded at this early stage.
you can stick your finger into the soil or buy a cheap moisture probe from a garden shop, if its damp underneath then its ok.

youll get a good feel for it as the plants grow, and youll notice that they need a bit more water a bit more often.