Yes, the adverb "ανάμεσα" (it is spelled with one "σ", not two) is always followed by the preposition "σε". For example:
Είμαι ανάμεσα σε σένα και στον Γιάννη. = I am between you and John.
Here we use "σε" because we can't use a definite article before a personal pronoun, but "στον" (σε + τον), because "Γιάννης" is a name (we always use articles before names).
Also, please keep in mind that "ανάμεσα" can be used for "between", as well as for "among". It doesn't matter if the people or things that are designated are two or more.
I hope that this helps. :-)
'among the cats' is also possible but that is only part of the sentence. If it wasn't accepted it means you had a mistake in the sentence. You should have made a report.
TIPS TO MAKE LEARNING EASIER + HOW TO MAKE A REPORT
And check out the Greek Forum here with more links.