The plants I sell come from diverse natural habitats all across the world and will of course find new homes in other equally diverse environments so to give a universal advice for growing your plants is naturally challenging, but I will try to offer some hints at least.
Substrate:
I typically grow my most of my rockery plants in more or less the same way with three layers of substrate:
1) Top layer - thin layer of coarse grit, rocks or pebble. This of course looks nice but has many important practical functions too. It will prevent all the more fine substrate underneath to float away when watered or blow away with the wind. This first layer will also lock moisture better in the soil below and prevent it from evaporating as fast. It will also add stability and an good airy support for the neck part of the plants that is often a sensitive and rot prone place.
2) Second layer - 1-20 cm in depth. Here you need a freely draining but moisture retentive mix, if you have a soil mix that works stick to that, no reason to experiment. If using soil or turf you will need to add some mineral parts such as sand, grit or pumice or similiar for drainage. Many, such as my self, use pure gritty sand and nothing else - this has proved to be a very good universal substrate for a wide range of plants as it gives both drainage and hold moisture reasonably well, making the thicker part of the roots less prone to rot. Plants tend to keep strong and healthy as well as looking nice and compact when grown in sand as well.
3) Bottom layer - Turf/peat/sphagnum or other moisture retentive material. Here the plants finer roots will find its way down to locate water without the thicker root branches sitting in wet soil where it will easily rot.
Light:
Basic light guidance is given in the plant description in the sales list, but most cushion plants and alpines in general as well as succulents grow in exposed places where they are facing full sun and wind most of the day if not all of the day and in order to keep their nice compact shape they will need to be exposed to a lot of light also in the garden. They will of course survive fine in shady places but the will quickly loose their shape and easily become etiolated and untidy.
Another thing you should think about in terms of light is that plants that are evergreen might be very sensitive to full exposure to strong sun in spring, especially if the ground still is frozen. For instance some conifers and Ilex might get burnt and potentially die from drying out but this can also happen cushions of Dianthus or Acantholimon too.
Water & nutrition:
Of course when newly planted you will have to water the plants for some time, best until you can see that they have started growing again. Once your plants are established you should not have to water them often unless you grow them in a much warmer climate. You will have to look at the plant also to understand their need for water, plants with spiky, thin, rubbery, thick, and/or glaucous foliage as well as of course succulent foliage will typically cope longer without water than those with for example many thin green leaves.
Of course the best way if you want to be on the safe side is to water the rockery frequently, as long as your substrate is well drained and light is sufficient this will have little negative impact other than the slight possibility of plants growing a bit to fast and thus loose shape. Equally with nutrition you should not need to add this very often, but if you think the plants stopped in growth or show signs of being weak and unhealthy it is a good idea to add some nutrition, bone meal is quite ideal.
Hardiness and heat:
Here is where it gets a bit harder. I would argue that hardiness by zones are very blunt guidelines and you can often grow many plants outside your zone if you provide the right environment or find the right spot. Normally it is the winters when plants are resting and spring when the start growth again that tend to be most problematic. Many rock garden plants spend winters in their natural habitat in either in cold but dry semi desert locations or in alpine regions where they ground are mostly frozen but the plants are insulated by a blanket of snow that will protect from wind and harsh sun when resting. In spring when snow starts to melt this gives a slow trickle of water perfect for gently waking the plants up.
Where I grow plants and in many parts of North Europe winters are generally quite unstable and fickle with temperature going back and forth around the freezing point and winter rains are more common than snow, sometimes on bare frozen ground. Under these circumstances cold hardy plants that come from areas such as Turkey, Kyrgyzstan or American desert plains will often have a very tough time surviving.
The substrate and drainage will play a large part in assisting plants to cope, but if you want to be sure you can of course cover the plants during winter with garden quilts in some suitable fabric or glass or plastic sheets. I keep selected mother plants in pots under cover in winter but the majority of pots and outdoor plants are left to survive with cover or any other pampering.
