@TheChemist77, per your request, as for the training, Coldsmoke mentioned what I do too:
A grower at another forum coined the phrase 'pruned for main stems'. Ideally you do that as little as possible and as early as possible (during veg).
Having said that, I don't entirely agree with daybreaker's assessment about you having too many lower branches that steal energy, it's not really that crowded to be an issue.
I suggest starting with training early and time the switch to 12/12 for your plant count carefully, to avoid the issue as much as possible. Either bend the main stem or top it, then prune the remaining main stems at the bottom IF they get very tall (which depends on strain, your nutes, your setup etc).
As for the lights, I honestly don't see the point of those fluos when you have 2x600watt on 5x5 already. Ideally you'd get a 1000watter than can boast to 1150, and use the T5 during veg (which will still give you short plants, which is mostly genetics, apical dominance).
That last term is the keyword here really, apical dominance, which you influence with training to avoid having to prune as much, and determines the structure of the plant. Apical dominance means a stem grows faster than the branches, and the branches grow faster than the branches of the branches. Stronger in sativas (long colas), weaker in indicas (where those side branches catch up with the main stem) and ideal in the best hybrids (every side branch a cola itself instead of golfball buds).
With a few wires you can manually intervene, much more effectively than those bottom lights do. I top and then spread out the remaining 6-8 branches.
Topped example, removed lower leaves during last week to reduce humidity:
View attachment 3303676
Older example, LST, basically bend the main stem early so it's at the same level of the branches:
View attachment 3303680 View attachment 3303682 View attachment 3303681
Calculate the desired number of colas in the space and divide that by the number of plants and you'll know how many main branches you want per plant. Differs a bit per strain (longer flowering huge colas opposed to shorter flowering smaller colas) but I can fit roughly 7x7=49 colas in my 4x4 space. More is doable, but gets crowded. With 6 plants I go for roughly 8 colas per plant, giving me 48 colas. That's under 600watt. You could try 8x8 for huge colas, or 9x9 for medium size colettas. With 28 plants you could try a much as 4 tops each. 16 plants with 4 tops each would work, 9 plants with 8 tops each too.
I hope this helps.