It is safe, but performance won't be the greatest. Lipos don't like low temps. The plane guys have known this for a while - even when they warm their lipos, the air flow cools them back down and they see a loss of performance.

About all you can do is start them off from a warm temp and see how badly performace drops off when they cool. This is one case where you don't want a lot of cooling air flow. Maybe you can insulate them to help keep them warm.

I went looking for specs and Kokam says their operating temp range is from
-20C to +60C. The low side is -4F if I converted right - I was worried that there was a lower temp limit below which they did't recommenmd running. Looks like you're ok there. I've never used mine in low temps - I just don't get into being cold while trying to have fun running an rc.