(You don't need to ask whether a character can like another character, it's your character, so you can do anything with him. It doesn't involve me.
In my opinion, the only thing you need to ask is if two cats can be mates. This involves both characters so both people need to agree to that.
As for Nightpaw liking him back, it'll really depend on how Fluffypaw acts towards her. You know what kind of cat she is, and she kinda dislikes Fluffypaw for his apparently parent-like acting towards her, and that time that he scolded her and Dawnkit.)
Nightpaw was a bit surprised by her sudden decision, and only remembered to say goodbye when Flamepaw was already gone. She stared after her for a minute, then started looking around for Darkkit.