Hello Maryjanenewbie-
You can absolutely use the bulbs you have throughout growth. More light in the right spectrum, regardless of the stage of growth, is a good thing! From what I can see the atinic blue puts out most of its light at around 410 - 420 nm's, which is very beneficial for plant growth and will help keep the plants tight- regardless of whether you're in veg or flower. Not sure about the 50/50 (would need more specifics), but I assume that if it is good for coral, it should be good for plants. Be aware though that all lights begin to lose efficacy over time and if the bulbs are over a year old, their output is going to be reduced.
IMHO you are severly lacking in the light department- so by adding the additional lighting, you are only helping. I would even go so far as to suggest you consider adding another 400 watt hid or possibly as an alternative, cutting your space drastically. Even by sectioning off part of your area you will greatly improve the amount of available light for your plants and significantly improve your environment for final yield.
Hope this helps- good luck!
