Yes, your six examples sentences are all correct.
ほか followed by no particle works as a conjunction. ほかの works attributively (i.e., modifies the following noun). This difference is important:
In addition, there is a cow. / Besides, there are (some) cows.
In addition to the cat(s), there is a cow.
There is another cow. / There are other cows.
- × 猫のほかの牛がいる。 (nonsense: "cows other than cats")
ほかに is tricky and it can work like both ほか and ほかの. There tends to be a small pause or a comma when it works as a conjunction.
は in your examples is just a topic/contrast marker, and it's often optional (see this and this for similar examples). It's particularly common in negative sentences. It adds a slight emphasis, but doesn't change the overall meaning of the sentence.