Dont put the tray directly on the heat mat or you could fuck your clones up.
Unless its got some sort of heat controler function i had a gecko heat mat and belive me i only used it once.if your room is warm enough you wont need it just dont water them to much and keep the humidity high until they have rooted and you will be fine.
defo dome on try to keep the temps around 23-26.
cut the end of the leaves off to help stop transpiration.
The best rooting hormone i have found was 95p wilkos diagonal cut is best because you show more i think its called pithium its the bit inbetween the outer skin and the middle part of the branch were the root grow from.
ive never done clones with rockwool though just compost and coco!.