I got two starter bonsai kits as gifts recently. I planted them all and had 100% success with growth in each pot. Now I am wondering what to do next with them in terms of light, water, soil, etc. because some were not doing as fruitful as they were a week ago. There is a total of 8 pots, with 6 different species: Jacaranda Mimosifolia (2x), Pinus Aristata (2x), Ficus Religiosa, Picea Mariana, Picea Abies, and Delonix Regia. Currently they receive about 6-8 hours of sunlight inside, where temperature is 72 degrees (we have AC on so it is constant). I have also started to play Jazz music for them in hopes it helps (studies vary but I like to think they enjoy it).

Current plan is to water when the soil is dry, then about a quarter to half cup each. I am not re-potting them until they get better roots. Do they need plant special plant food?

The other question I have is some look a little sad and maybe sickly. I have noticed two small flies, which I have tried to get rid of, but time will tell. Specifically, the Delonix Regia leaves are starting to wilt (Pic 1). I had another stem, but those leaves fell off entirely because the seed cap didnt fall off and eventually I had to slide it off. Second, the Picea Abies, one is turning brown stems (Pic 2). Lastly, the Ficus Religiosa stems are also all turning a little yellow (Pic 3). Should I continue care or move them or place them outside for better sun (live outside NYC)?

There is no tree that is 3 weeks old that can be called a bonsai. You don't have bonsai, you have seedlings.

72 degrees is far too hot. It's too hot for humans, even. A/C is normally not good for plants.

First thing you are doing is growing trees. It's going to take long before you can do anything bonsai related on them.
You have different species, all with different requirements. Explaining how to take care for all of them - and telling how to grow bonsai - is not feasible in a forum post.

Look up the care guidelines for each individual species. Let them grow. Then learn about bonsai.