5 and half weeks is cutting it close but you should be ok to do what you want to do in that time. If you were a well seasoned grower, it would be a non issue, but for a new grower, it can prove challenging. Genetics often decide the lower limit on when a plant gains sexual maturity but environmental factors and manipulation can affect that. The problem is that there is a trade off for every action done to the plant that doesn't allow the plant to progress naturally. In some cases the trade off is actually positive (in the case of high stress and low stress training) but in other cases it can be detrimental.
Doing plants in SOG is not necessarily a bad thing if the strain is the type to work in that method. However Sativa dominant plants are not made for SOG in my opinion. They are better suited to scrog because of their nature to become very "branchy".
There are people who have tried the 12/12 "forced flower" from seed germination, but unless you specific needs require such a set up, it is not the best method by a long shot. The trade off is that the plant doesn't have time to develop as it is naturally inclined to do, and because of that, you don't get the level of yield (nor the level of quality I believe) that is achieved by fully matured plants.
Your situation is somewhat unique because of your time constraints. However, 5 weeks is right on the cusp of sexual maturity(typically 5-8wks depending on strain) so forcing flower at that point may not hurt you, especially if you are doing scrog. If you keep them very healthy and then do your FIM around the 5th or 6th nodes, get them bushing out and set the screen when they reach about 12-15", they should do well.
I would say to set the screen over them and then give them about 1 week to begin spreading under the screen (with your help at continually tucking the growing tops back under as they grow up through the openings), or longer if you haven't reached the end of the 5 and a half weeks of veg, then immediately flip the lights(to 12/12) and cut the nitrogen rich nutrients in half and begin bloom nutrients. Continue tucking and bending the shoots and tops to keep them under the screen until you see the buds forming, then allow them to grow up through at that point.
Another little trick that encourages the plants to switch to flowering is to begin the flowering phase by turning off the lights for 24 to 36 hrs then set them to 12/12. This long dark spell will boost the switch over of the hormones that control the phases of growth. Its not a huge difference so if you don't do the dark period, it won't hurt anything as it works better with some strains than with others. It won't make it happen overnight but I have seen it work to quicken the hormone response for those strains who need more encouragement to switch over.
I do use coco in both hydro and soilless setups. I use about 25% coarse pearlite to keep the coco from compacting. I haven't used "mass P buckets" but when I do mine in soilless coco, I go from the cups of coco straight to my final 3gal containers. I would suggest that you use totes or the larger Kitty Litter buckets, as both of these work quite well. However, you will need to drill some holes in these and then set them in some kind of larger shallow tray to catch runoff. I ffound some plastic totes that held about 3-4gallons of coco which is cheap to get, and that works very well for me. You just have to wait until the plants are nearly root bound before transplanting to the bigger containers.
What kind of nutrients do you intend to use?
Green n gold grow mojo 
