Japan grammar: kara

Hi
Am trying to repeat some japanese grammar and got stuck on some basic stuff.

When using ~kara, for giving a reason, do you use short or polite form before the ~kara?

For instance:

日本語の先生は、日本人ですから、日本語で話します。

or

日本語の先生は、日本人だから、日本語で話します。

Next question,

Using ~to omou, to express that you think something.

If I want to express that I think something about someone else, do I need the 私は?

For instance

私は、山元先生はこわいだと思います。

or just

山元先生はこわいだと思います。

or

私は、山元先がこわいだと思います。

To say that you think teacher Yamamoto is scary.

For the first one, either one is fine.

山本先生はこわいと思います。
Is the most natural sentence that comes to mind, if you just express a sentiment about someone. (By the way “kowai” is an i-adjective, meaning the “be verb” is already in it, so you do not need any “da/desu” with it.

In contrast, if let’s say a group of students (including yourself) are talking about all the teachers at school, Yamamoto, Tanaka, and Suzuki sensei, and your friends says:
田中先生はこわいです。
And if you have a different opinion, you might say:
私は、山本先生がこわいと思います。
(by using the “ga,” you are pointing out it is Yamamoto-sensei, not Tanaka-sensei, and you add “I” to emphasize that it is “MY opinion.”)