I think you mean "flora". :-)
It is generally okay to winter sow most seeds of plants that can be expected to be hardy in one's area, not just alpines. Edit: And as Chelle points out below, many other plants too.)
(If you found a site that says "all alpines" will germinate at 20 C, the info there is extremely poor! Such a generalization should not be made, because the majority of genera require other conditions, e.g. stratification.)
Saxifraga longifolia is a monocarpic plant, which makes it less than completely satisfying for most people to grow. Hybrids of it, however, are not monocarpic - most people prefer perennial plants.
Alpines live in harsh places often with thin rocky soils, cold and strong winds. Yes, many alpines prefer lean conditions - soil that is low in organics and composed of coarse-grained material for good drainage. (So in other words, not "soil" as most people think of it.) The plants need to spend all their lives in these conditions to retain proper forms (or to survive in many cases).
Most people would plant seedlings or purchased plants in the rock garden or trough and allow them to grow to maturity there.
