Hey there Jesstar!
So I figured I would stop by and give my 2 cents on your questions!
So firstly; Which of your ladies are Auto's is escaping me right now... However I will say that you
CAN keep Autos on a 18/6 light cycle while they flower. They are going to do their thing regardless of you, so if you need to veg your Photo-period ladies longer I don't think it would be an issue if you kept your Auto's under an 18/6 light cycle while your Photo-period's finish up.
Going off that; If you are having issues lighting your grow space, CFL Light Hoods can be made rather easily and inexpensively... and (if made correctly) are super efficient. Not to toot my own horn, but if you need a quick example of one, check out the first page of my current grow journal, cost me under $100 and puts out 16,000 lumens. Another example that comes to mind is HunterNitro's vegging set up, even simpler than mine.
About cloning Purple; Totally up to you, it would offer you the opportunity to get a lot of hands on experience cloning, rooting, and revegging a mature plant, which is extremely useful. Plus you'll have a new plant ready to go for your next grow!
About Co2; Basically the big hype here is that a enclosed Co2 system offers your plants a much more ideal environment to grow in, meaning healthier (better) plants. This is because Co2 (promotes growth) is constantly being released into the grow room (and in an optimal grow room, there is enough ventilation to move said Co2 around) and is replacing the used air that the plants are producing...
Going off that; A true enclosed Co2 system isn't very realistic in your set-up without completely enclosing your space and achieving a negative pressure system inside...
HOWEVER there are smaller much more economical Co2 set ups available.
Such as this one...
They run anywhere from $20 to $40 and can be found online or at grow stores, you just let it sit in the corner and do its thing! You might look into that if you want to experiment with Co2.
On your Gnats; I'm sorry you have to deal with those little *$&#er's.... You seem like you know how to fix the problem, however I would definitely find a small corner pharmacy/grocery type store and find some 3% First Aid Hydrogen Peroxide. It is pure Hydrogen Peroxide and Purified Water, totally okay for your ladies.
Here is what I have in my bathroom cabinet.
Keep applying how you have been. You also might consider a layer of Gardening Sand to put on the top of your pots instead of that airy perlite. This will stop the mother Gnats from burrowing and laying eggs in your soil. Which you should only be watering when it is dry.. Gnats absolutely love wet soil.
And Lastly! Molasses; This stuff is awesome, Unsulfured Blackstrap Molasses is often used in Cannabis Horticulture and helps naturally sweeten the flowers, as well as providing essential minerals. Most name brand flushing agents have basically the same ingredients as Blackstrap Molasses.
It's use varies widely and each grower has their own uses for it. Some use it starting in Veg, some starting in Flower, others not until Late Flower, some in between Nutrient Waterings, and some only for Pre-Flushing/Flushing...
I am not sure the effects it will have on your Gnat problem unfortunately, however if you do get that under control... I would definitely recommend trying Blackstrap Molasses in one way or another! It is usually dosed anywhere from 1 TSPN to 1 TBSP per Gallon of water.
I hope this helps, I am still learning new things all the time and sharing information I have gathered helps me do just that! You're doing a great job man, keep it up.