From a fellow grower: Some of my plants are thin and spindly, grown in heat controlled and UV lit partitioned area of my garage. Most times when the weather hits the sixties I give them outdoor hours for natural light.
I use your first level of nutrients mixed in a gallon, then into a small spray bottle. I spray just the soil mix and not the seedling itself until the soil is damp once daily.
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Most growers go with ~200 watts of quality lighting per plant to get through flowering. Your plants are stretching because they are hungry for more light.
Seedlings don’t need nutrients. The cotyledon (little round) leaves have all the nutes the plant needs until the cotyledons yellow and fall off. If you are using a good cannabis soil, then you won’t need nutrients for 4 to 6 weeks.
You’ll want to go with one plant per pot or the plants will compete for water and nutrients. The plants also may mature at differing rates and you’ll have a hard time managing them.
Cannabis needs wet/dry cycles for root health.
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Agree with @MidwestGuy guy and would suggest a transplant on the stretched seedlings. Transplant them to their forever home and bury them to about 2” from the small cotyledons. Maintain 78-82 degrees and 65-75% RH 
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