Why does cats here become "Kēla" rather than "Kēli"? Is this just an exception to the rule? Also why is "kostōbi" used as I thought the adjective ending is the same as the noun question? Sorry for the benign questions. I'm just a bit confused. Thanks!
Not every noun takes -i in the nominative plural. Nouns of the third declension (that is -o, -os, -on, -or) or the fifth declension (-i, -is, -ir) have -a in the nominative and accusative plural. See http://wiki.dothraki.org/High_Valyrian_Noun_Declensions for more details.
Adjectives take the same gender, case, and number (though the last item is complicated by factors the course has not yet taught) as the noun they modify, but not necessarily the same ending. The noun follows its paradigm, and the adjective follows its own, and if the endings happen to be the same then great! But this is not always the case. See http://wiki.dothraki.org/High_Valyrian_Adjectives for more details.