-nothing right now. Too small for anything else really, you don't want to burn her. I'd just add some superthrive, and a little b1 (1/2 tsp or less per gallon) to every watering until she's about 6-7", then introduce your nutes at 1/4-1/2 strength.
-I've never used the seedling mix, but have used the black gold coco water hold last year and this season, and I really like it, got GREAT results with it, and the nutes I used. -good stuff.
Good luck, and post some pics of the progress.