no you are right its good to keep both lights in. Just make sure you set ur tent up and get the "weather" dialed in before anything else. I made the mistake and germed some seeds threw em in the tent. I have a secret jardin tent so it has really good quality reflective material and a 400w sunleaves hid mh/hps light. Well i thought my WindTunnel 4" exhaust(running from light to outside of tent) would be enough to combat the heat..wrong. I had to quickly purchase a window unit. But i would definately use that light setup all the way through the grow to avoid any stress from changing the light mid grow and causing hermies or stunted growth basically just eliminate the possibility of that from the get go. Dimming would be a great idea tho.
As said previously the just need moisture and heat to get started. But the whole thing with the light is HIGHLY debateable and mostly oppinion with a little bit of fact. Most ppl say blue is good for veg. I used a hortilux blu mh and wow it definately treated those ladies well. And red (hps) is good for flowering as it increases yeild by density although ive heard blue isnt as dense but has more trichs and clusters, etc. So ur light would be a perfect balance and imo i would use that the whole grow and not change it at all other than turn the dim off once its established. As you near the end of the grow i would gradually dim again as again ive heard and seen many results proving that a good 48 hr dark period forces plant to try to protect itself with thc basically. So..its up to you ultimately and finding what works best for you.
As for nutes i strongly recommend using general hydroponics GO box(general ORGANICS). its really cheap; only like $30 and u get several bottles of really good clean potent natural nutrients that will feel ALOT of plants. Keep in mind what u put in ur blood, lungs, brain..all these are vegan(plant extracts) except the processed squid so there is not room for harmful products feeding your ladies. BUt i recommend due to its easy for beginners because its hard to burn the plants. Dont let this fool you though the nutes are really strong and the plants will be VERY vigorous(i accidently overvegged in a couple of days due to growth rate lol) but i believe doesnt burn ur ladies due to it being all plant extracts. So you can feed the fuck out of em
the box recommended once a week i feed every 5 days. THe box is also good in the sense that the smell of plant grown organically will always be 100x better. Experts will also find this box fun as the nutes are broken into about 7 bottles instead of 2 or 3 bottles of a bunch of stuff premixed. This allows you to better control a problem should you have a problem with burn, deficiency, or whatever. Basically u have more specific control over your plants diet. But they make it very user friendly for beginners. Read the box AS WELL AS the bottles. Buy a ppm meter. I would strongly advise to play with the variations possible to ensure u find what works best for you. I used the example elsewhere but say u could use more nitrogen but want the squid as well in the diet. Increase the bio weed nute decrease the squid. All while keeping a specific ppm range for your daughters as well as covering the whole food pyramid. mm thats alot to take in but thats my advice take it or leave it.
Oh yea before i go also when they pop ur gonna want as much light as possibly without burning, stressing, or bleaching them. When the break ground i would say turn it up just a little each day until full power. But in my oppinion you wouldnt want to take long doing this as your plants will be stretchy if your lights not close enough. I use 400w though so im not sure how close your 600 should be for seedlings. just look around the site you should be able to find someone else thats already asked lol