My Phrag Sorcerers Apprentice came in Rockwool & pearlite , like you i,m not familiar with it,s use.Plus the latest growth on the plant was about 1 1/2" above the surface of the rockwool & i wanted it to be @ least level.
Repotted into s/h using LECA & about 10% charcoal(why not !) into the same pot it came in & just sat in a deep saucer of rainwater.
Never missed a beat.I have a spike about 2' high & 3 new growths(all growing).
The 3 seedling Phrags took a while to get comfortable(there in s/h pots) but now are growing well.I,ve had them for about 2 1/2 months.