follow instructions on label... i'm sure it's fine. I'd holdoff on fixhmix the first couple weeks, possibly longer. You are in soil. You won't need to add much initially. The seed will power itself for 10-14 days or so. The soil should last a while beyond that. This depends on how hot the soil is.
After that intitial period, you'll want to slowly ramp up fertilizer to maintain nutrition levels in the soil.
Light intensity.. let plants dictate. If stems of seedlings are stretchy or pale, they need more light. If they fall over and not from stem weakness, that's too much light. If they get weak or fall over, prop them up with a toothpick or soemthing. It doesn't have to last more than a few days at most. Adjsut light accordingly.
A problem i see is that you ahve seedlings in a large pot. This makes watering difficult. When you do water, make sure it saturates a column all the way down to bottom of pot. You want roots reaching down and not superficially for roots. A wet-dry cycle is also needed. This is more difficult when watering a small area around a tiny plant. it'll be tough to judge when to water and how much.
by 14+ days, i'd water that entirity of the pot with runoff to ensure no dry pockets. wait for top 1" to dry before repeating. learn weight of dry pot. Shouldn't take more than 7-9 days for next irrigation. It gets much longer than that you grow more microbes and such with a constantly wet substrate.