"One" applies to a feminine group? but there are other feminine words besides women, couldn't they be girls? This is the feminine version of the pronoun "they", you cannot replace that with something more specific. Sorry when we translate to English that information (that they is feminine) is simply lost, just as when we translate the many forms of "you" from another language and they all become simply "you". In English, that information was just not considered important enough to keep separate pronouns to distinguish between. (Once upon a time we used to have the singular "thou" as well as the plural and formal "you", but now we just say "you".) Scroll all the way down because "one" is also used for animals and things and "oni" can only be used for people with at least one male person in the group: https://www.duolingo.com/skill/pl/Basics-2
Singular is "ciasteczko": http://sgjp.pl/leksemy/#11092/ciasteczko
In order to find all the forms of any word, you can use http://sgjp.pl/leksemy/ - if you write any form of a Polish word in the search field, you can get all the forms thereof, sometimes various depending on variants of meaning of that word. But this dictionary gives only forms of a word, sometimes with just a very short note on the meaning - you often need to use another dictionary to check the meaning, e.g. https://sjp.pwn.pl/ or https://wsjp.pl/
One • Jedzą • Ciasteczka • ↢ • ↣ • One ( They - Pronoun - all groups not containing men • Ona nominative plural • From Proto-Slavic Ony, from Proto-Indo-European h₂eno- ) • Oni ( They - Pronoun - 3rd Pers Masc • From Proto-Slavic Oni, from Proto-Indo-European h₂eno- ) • Polish ProNouns
|♀ ♂ ↓||Num ↓||Nom||Gen||Dat||Acc||Ins||Loc||Voc|
|♀||Plu||One||Ich / Nich||Im||Je||Nimi||Nich||—|
|♂||Plu||Oni||Ich / Nich||Im||Ich / Nich||Nami||Nich||—|
I am sorry, but I think, you are wrong with the length of stem here, is should be "ciastecz-ko" , the same as with "ciast-ko". The digraph "cz" in Polish is one sound, so it should not be split.
It is the pattern of declansion no 115* according to prof.Tokarski, as with "jabłko" - see the list of nouns by declension pattern here http://grzegorz.jagodzinski.prv.pl/gram/i_deklin04.html and the list of patterns here http://grzegorz.jagodzinski.prv.pl/gram/i_deklin03.html (sorry, you need to deal with the annoying ads here, e.g. I use uBlock)
Diminutives exist in English too, even though they are rare.
- Pig -> piglet
- Leaf -> leaflet
As you can see, the diminutive is a "small-version-derivative" of the original noun with a similar meaning. But it's impossible to guess what the exact meaning will be after it's diminutivized.