"The cats sleep."
Yes but its often dropped, at least in speech. Too much of a newb to say about writing.
We now accept 잠잡니다. Please report if your answer should be accepted. Thank you. :)
No. 고양이 is the actual word for cat. 들 makes it plural and 이 makes it the subject.
Would 고양들 ever be used instead of 들이 in common speak or not because of something I don't understand yet?
고양들 would never be used because the base word for cat is 고양이 so you're not going to change that 이. if you were trying to ask about 고양이들 versus 고양이들이 instead and just missing a syllable in typing though, I don't know an answer.