Sorry the image is crooked. Posting from my phone.
This is the progress my Poinciana has made in just 3 days from barely pushing soil to this.
Should I begin to look at the seedlings I want to keep and repot or just remove the ones that I won’t be using? I feel they’re still too young to safely move. And do I need to trim anything or allow them to grow to a certain size first?

Do not see them as bonsai for now. The only thing thing you need to do now is purely horticultural: repot when necessary into a bigger pot, water, fertiliser; thats it

lucR wrote:

Do not see them as bonsai for now. The only thing thing you need to do now is purely horticultural: repot when necessary into a bigger pot, water, fertiliser; thats it

I definitely don't see them as bonsai yet. I am not really entirely familiar with tree growing though, I usually deal with flowers and vegetables. Just want to make sure I am doing what I need to and that I am not going to kill them or inhibit them. It looks as though the 2 tallest are the ones I will save, as of this morning they have about 4 true leaves starting; so I should be able to just remove the others to different pots and see how they grow as well. I did pull one of the smaller ones that was starting to show brown on the cotyledons and it had a fairly substantial taproot already, so I imagine the taller ones will thrive if i remove competing roots around them.

My question dealt more with the trimming and maintaining a good environment. I know removing the competing roots will help, but do I need to trim leaves (either true or cotyledons) to encourage more growth?

I would leave them all as is for a long time. You just don't know which will live and which will die.

The shorter ones may be weak, or may just be late bloomers.