1st grow and in growing more than i should be but ...

"I'm going 18-6 all the way through until the last 2 weeks, then I'll flip to 12/12". It won't work, because as you said, the plants are not autoflower strains. This means that they won't flower unless you put them under 12/12 light schedule. You have a couple of alternatives: #1 Wait until the canopy is full, then switch to 12/12, although the oldest plant will use most of the tent. #2 trim branches and defoliate the oldest plants to give the younger ones time to catch up. #3 try to set up a separate flowering space for the oldest and bigger plants