The point is with fresh soil they shouldn't need nutes for a while but if you only need a bit of soil to fill the new pots it won't last so long so after a couple weeks you should probably start feeding. If going from a small starter pot to a multi-gallon sized pot then they don't need feeding for a lot longer. The size of your plants in relation to pot size has a lot to do with when they should be fed or watered. Soak good, then let dry before soaking again.
Clay pots aren't the best thing. They wick a lot of water out of the soil unless they are glazed and can dry out fast. I just use plastic pots and the biggest I ever use is 4gal.
Just use 1/4 of the max for the first feeding to see how they like it then go up from there if they look hungry. Any sign of yellowing lower leaves means they want more if you're seeing them get taller day by day and especially if the centers of the the grow tips are lime green until they get larger. That means they are growing fast and it can take a day or more for them to "green up".
If space is an issue some square pots will maximize the amount of dirt in the pots and fit snugly in a tighter space.
There's no specific time you HAVE to switch to 12/12. That's up to you. You want to give them some bloom nutes and any bloom booster at that time but they need lots of N too as they go thru the stretch for about 3 weeks. They eat up to 4X as much during the stretch so that's when they need it all.
When I repot I saw the bottom inch off the rootball with a bread knife to get rid of all the stringy roots at the bottom and shave down the sides if they are wound around there too. Then the roots branch out like when you top a plant and the side branches grow out faster. At the end of the grow the whole rootball is full of fine roots and there are no long ones at the bottom at all. Repot when the plant is at the point of needing water so the ball stays together.