Nope, this is not an idiom, just some sentence about a mouse among some cats.
That's what I thought. When I thought it was idiom I was trying to figure out it's meaning but didn't make much sense except to maybe illustrate a rookie among professionals or something related to "a wolf is sheep's clothing".
I understood it as "a sheep among wolves": scared, defenseless, odds stacked against it.
Both among and amongst are correct: http://blog.oxforddictionaries.com/2013/02/among-amongst/
Is there a need for (de) before (cats)? A mouse among(st) cats but here (de) is added any reason?
Tussen, zwischen and between all come from proto-Germanic twiskas (two-fold, double) as evidenced by the t(w) z(w) of twee, zwei, two.
English still preserves the distinction between between two people or two groups and among(st) several people or several groups. When I saw Een muis tussen de katten my brain said "Oh, there are two cats - that means use between". Then I saw among in the answer. So, does this mean Dutch does not maintain the between/among distinction?
Even in English there is not always a clear difference between between and among(st): https://blog.oxforddictionaries.com/2015/06/29/grammar-myths-among-or-between/