If you re-pot the sedum the new soil should have sufficient nutrients without worrying about fertilizer.
For your sempervivum potting mix, you can add some of the gritty sand with the potting soil. I am not sure what the difference is between light and heavy potting mix?
You can even add some of the grit to the mix. You can top dress with the grit or pea gravel/small rocks.
You can pot up the entire colony as is, or separate and plant separately. Just depends on what you want to do.
The individual rosettes will grow larger is separated and given room to grow. But it is also interesting to see them grow in a colony.
