I vegged under a HP sodium before. It is possible but the sodium vapour give off more of the red spectrum, heading towards 2700k, for blooming.

The sodium vapour does give off the full spectrum and you will have a enough light in the region of 6300 - 4000k which is best for vegging.

It would be more economical in the long run to get a metal halite for the veg and the plants will love you a little more

When you took the plants indoors the 125w CFL might not have been enough to support all 4 plants. When the leaves don't get enough light they will yellow, dry and drop off.

18/6 is good for veg. Be sure you keep an eye on them just in case the light might be too close or even too far away. Try to keep the node lengths at 3cm and you should be ok.

Be careful moving plants from indoor to outdoor when the weather is is very sunny outside. This can damage the plants as they are not used to the outdoor environment.

Ease them into their new habitat by giving them just a little sun each day.