In mature Quenya, is the inflectional stem used to form the plural cases as well as the singular ones (and do we have attested evidence of this)? When one looks for example at the entry nén (nen-) at Eldamo and its attested cases, it seems that Tolkien used to be of two minds about this (even regarding the singular cases!).
E.g., should the nominative plural of súrë (súri-) be súrir or súri ?
Inflexional stems in Quenya are applicable to all instances where a suffix is added to a word. They mostly arise due to different historical developments of certain sounds in word-medial and final (or near-final) positions, so any kind of suffixation works the same here.
An attested example is pilin "arrow", nominative pl. pilindi, dative sg. pilinden. Be you will certainly find contradicting forms in Tolkien's notes, whether due to him experimenting with different forms, or due to multiple historical layers within the language (and Tolkien experimenting with those).
It is a generally good idea to be internally consistent when we try to use Elvish, so my rec0mmendation is to use the stem forms for all forms with a suffix, even if Tolkien at some point wrote down a different form.
For súre, the plural is súri - the pluralising suffix -i merges with the i of the stem form in this case.
When in doubt, check out Quettali for a useful tool for conjugating and inflecting Quenya words. It has gone through several years of review and refinement and should be (on average) quite accurate.