Since you've been using nutes already, there may be a danger of salt lockup* in your soil. What I would do if those plants were plopped on my doorstep:

1) More light. (You're doing that already)

2.a) Let the "weakest" looking one dry out a day or 3. Pop the root ball out of the cup and check development. Should be some threads up against the cup. Could be none. Could be a mass. Put it back into cup and into cycle 2.b

2.b) This is a 1-3 week process. Water the remainder of the plants with clean, fresh water to the point where water is running - not dripping - out drainage holes. Let dry 2-3 days, or until soil at bottom is verging on dry. (I use a wooden chopstick) Repeat.

3) As each plant reaches the point of full 5-finger leaves, transplant into bigger digs. Do the dry 'em out thing for a few days before xfer, makes it real easy to pop the roots out in a mass. Don't overwater when putting in the new pot, just enough to soak the top 2" or so. (It'll sink, and the roots will go down looking for it)

4) When plant is acclimated to larger pot (3-7 days) start whatever nute schedule is planned.

*salt lockup is when chemical nutrients dry out and leave chemical residues on chunks of wood, loam, etc. in your soil. If your roots hit these chunks of salt encrusted stuff, you will get "nute burn"