"The small, grey mice are in the kitchen."
Translation:A kicsi, szürke egerek a konyhában vannak.
It is OK here, as "kis" is in front of its noun, followed by another adjective.
But if you inserted as much as an "és", it would change to "kicsi":
"A kicsi és szürke egerek... "
I see, this makes sense, it's about proximity to the noun. But can one say "A kisci, szürke egerek a konyhában vannak." ? Or is it required that there to be a word in between?
No, it is perfectly fine. "Kicsi" can replace "kis" mostly anywhere. Well, maybe not in compound words, because it would sound unusual, those are kind of set forms with "kis-". But sometimes even there it is used.
Exactly. Or, when you do that, and you can very much do that with "kislány", the word gets split into two words: "kicsi lány". It is a more endearing version of the word.
Which immediately brings to mind this short video. Barely more than two minutes:
Part of Hungarian culture. :)
And another example, for "kislány":