Yes, you can use this fertilizer with Fox Farm soil.
They don't look bad for their age. the size is normal. but it could have been better.
you have a moisture problem. she is very skinny. should be at least 50%. at best 70%.
dark leaves indicate that the roots are drying out. try to water so that the whole soil is tested and not just in the center. Basically, most of the roots are located around the perimeter of the pot. and due to low humidity, these areas dry out faster and, accordingly, the roots that are there.
let's sum up.
raise the humidity to 50%+ and make sure that when watering, water penetrates well everywhere. and make sure the soil doesn't dry out.