dynamite as the 18-18-18 would do good, maybe add a bit of micronized iron to the soil too. They love the heat, that will help them grow quickly.they get HUGE thick roots and can easilly push themselves out of a pot. They look good for seedlings, you have quite sometime before blooming though

They will do just well with the heat, as long as you get them enough water.
I don´t think they NEED fertilization as a matter of life or death, in fact, they will survive in poor soil, but don´t expect them to be very happy.
Good luck.

thanks for the replies : )

How big of a pot will I eventually need for these plants?

They seem to be in a larger than necessary/desired pot for their young age. Seems to be more soil space than root mass. Not positive, but don't they prefer to be potbound?
Ultimate size pot would depend on whether they are standard-sized orange, dwarf orange, or white 'birds'. Best to consult a horticultural professional at a garden center, or your county extension service staff.