Geez... that Green Crack, she is having a very bad hair day. Sometimes some of those scrawny freaks will have extra sticky and potent little buds on them, but I wouldn't bet on it. Probably you have the right idea to move her into the little tent and put the Kush under the 1000.
I flowered for a few years with 3 gallon pots. I would put them into flowering at about 14-18" height and they'd end up at around 25-32" high after the stretch, if that gives you some idea of size.
I grew about 9 plants per 600 watt light, with a 4' x 3'2" growing area per light.
Now, my grow, as you may know, revolves around me being away for most of the week. So pot size is 95% based on managing the watering schedule. I want them to go for 5 or 6 days before drying up. In three gallon pots this was difficult, especially for the taller plants of the bunch. At one point I tried some of those same size plants in five gallon pots. They were too big, wouldn't dry out enough before the weekends, and I had all sorts of deficiency issues from them being wet too long.
Now I grow in 7 and 10 gallon pots. (The ten gallon pots aren't 100% full and I figure they're holding about 8 gallons). Four plants per light in the same size areas I mentioned. This size area is pushing the limits of the 600 watt light. It seems a tad dim around the edges. Maybe with a larger reflector I'd cover slightly more area but I doubt I'd gain much.
My total grow area under each light is roughly 12.64 square feet. Exactly half of your 5'x5' tent. Each plant covers 1/4 of that space so has a canopy size of 3.16 square feet. Easy to measure this as they're in scrog screens.
Based on this gibberish, lol, I wouldn't expect you to cover a 5x5 space with a 600 watt light. A 1000 would be ok. Two 600s, or the 400 and a 600, would be better.
There used to be a guy on 420 who grew commercially and sold to dispensaries. He grew large plants, yielding over a pound each. He vegged under t5s only- so I'm sure that this works fine.
As for the transplanting, I've never experienced any problems transplanting whenever I felt it was needed. Quite the opposite.
My sense though is that the plants won't be putting out much root growth after the stretch, and I wouldn't expect them to have time to fill five gallon pots well. Not that this would be the end of the world- but 5 gallons seems a bit large. If they're itching for a transplant - 3.5 - 4 gallons of soil in the 5 gal pots sounds about right.
I don't really know the size of these plants you have in 2.5 gallon pots, but I suspect they'll be fine left alone. Like you said- it comes down mostly to the watering thing IMO. If you're around enough to water the little pots every 3 days or whatever they should be ok. In full 5 gallon pots you may be waiting 8 days or more between waterings which is pushing it.
Hmmm... Molasses. People usually use it up to the last week or two. Claim it does wonderful things. I use it on and off, haven't done it scientifically enough to decide the difference.
Geez- not sure if any of this helps or makes sense but hit me back if it doesn't, or if it does...