With that type of space i would look into cfls or some t5 floros.I would also recommend some autoflowers since its a small space but you can get mostly indica based strains.That way you can have decent yeilds with small plants.
I just throw my seeds in a cup of water when they sprout i put them in dirt or rockwool and leave them in the dark usually within a day it will sprout up.Then i would add the light but like you said everybody has there own way.
I would go with the dna sharksbreath,sour cream and cole train.The mixes sound crazy plus im a fan of dna and reserva privada.I grew the kushberry and connie chong and both were great.The kushberry was better imo.
You can use gel for a better chance of them rooting but you can just place them in the rockwool(make sure no air gets in) place them in a dome and spray from time to time should root within a week.
No i would not bother.Plastic rubbermaids with a 400 watt hps sounds like a disaster waiting to happen.Hey who know someone probably done it already but i dont think its possible.