It would help to know where you are located and what your climate is. Also, will the plants be indoors or outdoors? Are you growing seeds only, or also rooted plants?

There are many opinions and many options for a potting mix for succulents, but the biggest factor is your growing conditions. Good for you on your efforts to have everything ready in advance!

Perhaps Scoria, which I think is also known as lava rock, would be a good substitute. I have trouble finding it with small enough particle size, but you might have better luck. Or pumice might be good, or perhaps chicken grit? I think any gravelly gritty stuff would work as long as you get the right particle size. Which is easier said than done, sometimes.

I live in the baltics, the weather here is changing every day. We can get a day full of rain and a day full of sunshine. I bought a sempervivum hens & chics mix yesterday and got 17 pups out of 2 plants, 3 of them did not have roots. I planted the seeds in 1 part perlite, 2 part peat moss and 3 part potting soil.

Well considering your climate is probably pretty cool, I would go for minimal (if any ) organic matter for most things. I think the mix for your sempervivum might retain too much moisture. But you might be able to get away with it over summer.

May I ask what is 'cucumber soil'?

Most of us do not use vermiculite. And prefer not to use peat. The recipe you found is good; as mentioned on the site you linked to, Al (tapla) is long-time member of this forum (this used to be Garden Web, bought up not too long ago by houzz...-:( They posted a link to his post about Container soils - hopefully you had chance to read it. You don't need to make mix exactly as Al suggested, but -if the concept is understood - make one from materials available that would perform same or very close to it.

Many ppl use just C&S soil and perlite. If so, they mix it in minimum 1:1 ratio, and often use more than 50% of coarse/sifted/rinsed perlite to achieve good drainage. I wouldn't use extra peat, there is usually plenty of it in C&S soil.

Many of use mostly (or 100%) inorganic mix. Grit is very good ingredients, and hopefully you find it. We get poultry - chicken grit, that is easy to find in farm supply stores and quite inexpensive. Make sure, if you find it, to get one without addition of crushed shells...

It would help if you posted photos. Sempervivum 'chicks' do not have to have roots when divided - they will grow them soon. But it is not necessary to divide them...they naturally grow forming big clumps, and often look very good like that. Obviously, it is your choice and you should do as you like...

May I ask what seeds did you sow?