yes, you can flower under cfls in 12/12. however.....

1) the hps will probably provide many times more light for it.
2) i assume your cfls are cool white/daylights, and ideal cfls for flowering are the warm whites.
3)yes, your yield would reduce slightly if its deprived of optimal spectrums and levels of light, but not by much over a week.

if you havent begun flowering yet, id suggest you wait for the bulb. that extra veg. period will only help to increase the yield anyways...:rasta: