The two distinct meanings of Süße — sweetness and cutie — have been formed grammatically differently from their common root süß (the adjective) and thus function differently. However, both have plural forms.
Süße meaning sweetness is a nominalisation formed with the -e suffix. If possible, there would have been an stem vowel umlaut, as can be seen with Größe and Länge that were formed similarly. Typically, these forms indicate a quantifier. If you are just talking about a single object, these quantifiers cannot be pluralised, but upon comparing the quantifier of different object, pluralisation makes sense. Hence, die Süßen (plural of sweetness) is rare but possible.
Milchschokolade und Eiswein schmecken ganz anders. Ihre Süßen lassen sich überhaupt nicht vergleichen.
When Süße is used to mean cutie, i.e. as a term of endearment, it is actually only an adjective that assumes the syntactic role of a noun in the latter’s absence. Thus, it still declines as if there were a noun following it and even assumes that invisible noun’s gender. Depending on whether a definite article is preceeding or not, strong or weak declension must be observed.
However, due to the overlap with the other full noun (sweetness) not all forms sound idiomatic. While technically forms could be used for all numbers, genders and cases some just sound wrong without some other quantifier. Compare the following examples:
Ein Süßer steht an der Bar.
Eine Süße steht an der Bar.
Ein Süßes steht an der Bar. (Slightly weird; probably sounds like a child standing there. But could technically also be used for ‘sex unclear’.)
Der Süße hat sich einen Cocktail bestellt.
Die Süße hat sich ein Cocktail bestellt.
Das Süße hat sich einen Cocktail bestellt. (See above)
Hier steht der Drink einer Süßen.
Ich bringe einer Süßen ihren Cocktail.
Ich spreche eine Süße an.
Hier steht der Drink der Süßen.
Ich bringe der Süßen ihren Cocktail.
Ich spreche die Süße an.
(Mehrere) Süße stehen an der Bar. (Sounds weird without mehrere.)
Die Süßen bestellen sich Cocktails.
Since this construction is ‘only an adjective’, many dictionaries will not list it separately.