Translation:I like dogs, but my sister likes cats.
IMO it's mandatory here. While the comma before some coordinating conjunctions is omitted…
„Ich gehe zuerst und du folgst mir.“ – „I go first and you follow me.“
…when the conjunction implies a contrast or difference such as aber, doch, jedoch or sondern, there has to be a comma, even for coordinating conjunctions:
„Ich gehe, aber du bleibst here.“ – „I go but you stay here.“
die Katze (plural die Katzen) is grammatically feminine word for the biological family of animals and it can both mean a female and a male cat.
der Kater (plural die Kater) is grammatically masculine and means specifically a male cat.
If you explicitly want to talk about a female cat, there is the grammatically feminine word die Kätzin (pl. die Kätzinnen), but IMHO it's not used that often. AFAIK the usage of adjectives is more common here:
„eine weibliche Katze“ – “a female cat”