When is the earliest I can use hair concealer fiber like Caboki/Toppik after hair transplant surgery?

It is best to wait about a week or until the scabs have healed and the grafts are securely in place before applying a topical to the scalp. It can be difficult to wash the Toppik out so it is best to wait until the grafts are in place so there is no chance of rubbing them out when washing off the Toppik. However its always best to discuss with your hair transplant surgeon.

You can use concealers a few days after surgery.  However you should check with your doctor before doing this.

The earliest you should use those products is two weeks after your surgery, when it is certain that your scabs/crusts have all fallen off. Once that happens you can use pretty much any sort of styling product that you'd like.