light mix is nice to start out new seedling in, as its not to hot for a small seedling plant
I dont know the royal or the grow mix, but I know the batmix and its nice for weed, but also a bit hot, so if it was me with them two I would use light mix first 2-3 weeks in Cup`s then repot em to a bigger pot and make a mix of lightmix and batmix 1-1 and for the final pot I would only use batmix (just one way to do it)
in general, small plant dont need much food, so go with a light fertilized one to start with, later on when you have bigger plant (mature or in flowering) they can take much more
dont repot in flowering, do the final repot a week before
and most important about soil for weed is that it have the right PH, some cheap "super market" soil`s keep to much peetmoss and have a bit low PH (5,5 to 6,2)weed need a PH between 6 and 7 so look for a soil that is some where around that (the one I usually use is 5,8-6,6 and do the job nicely
beside PH the other most important thing is to keep it light and airy, so keep a good amount of perlite (10-30%) in the mix, stuff like coco fibers and lega stone/nuts (small clay balls) or green sand is also stuff that can make a compact soil a bit more light with better drain, and drain is so importent, weed dont like to stand in "Old" staled water for to long (it need oxygen to its roots) so make sure the pots you use all have nice drain hole`s
and start em out in Cup`s or small pot`s and move your way up, a small seedling in a 5 gallon pot is a bad combo, almost impossible to water probably with out over watering (need some experience here) I use Cup`s for the first 1-3 week`s then 1 gallon for another 3 week`s before I use a 3 gallon to finish of veg, a week or so before I start 12/12 I repot in to the final pot, a 5 or 7 gallon depending on how big my plant is