SKOS collections are intended to represent groups of closely related concepts within a particular thesaurus. SKOS Primer provides an example of the animal milk collection, which contains cow milk, goat milk, etc. Thus, this is an additional organizational feature (orthogonal to the normal hierarchy of concepts) for related terms - the idea is that “milk by the source animal” is not a concept in itself — a broader concept of “cow milk”, “goat milk”, and etc. it will still be milk.
Conceptual schemes have a different intention: they are designed to capture / identify a single complete thesaurus / taxonomy, which is useful in situations where several such thesauruses / taxonomies coexist. Elements in ConceptScheme are not necessarily a “grouping” of closely related terms, but they are all part of the same general hierarchy of terms.
As applied to your examples: the product range diagram seems to be best suited to define a “single complete taxonomy”, so it can best be used with ConceptScheme.
As for the products belonging to each customer: I am not sure if I will represent this using the collection at all (you could just have an identifier for your customer and just have a "ownsProduct" relationship for each individual product). Even if you did, the SKOS collection is not the best fit: it is more situational knowledge and less related to the thesaurus / taxonomy itself. But if you need to group certain products together in your product range diagram, for example. "Toothpastes to taste" (with the subtypes "sweet tasting", "salt tasting" and "almond tasting"), this is what you would use for the SKOS collection.