grammaticality - Mixing past tenses in the same sentence

I have a question about violating verb tenses. I was taught that you can't use present perfect (or continuous) and past simple within a sentence. You either have to use present perfects (or continuous) with present perfects (or continuous), and past simples with past simples. But I've found this not to be true, so I'm a bit confused.

Here is an email one of my native American friends sent me.

Hey, it's been such a long time since we last emailed each other. Thanks for sending me an email! Getting that email was such a pleasant surprise, because I was just thinking how I've been wanting to send you an email as well.

What confuses me the most is the last part. How can you say that you were thinking about how you have been wanting to do something?

I was taught that this is wrong: "he has decided to go hiking, so I went hiking as well." But I'm assuming that my friend's email is correct, since he IS a native speaker.

So, this is what I'm thinking: I can't say present perfect (or continuous) + past simple, but I can say past simple + present perfect (or continuous).

This violates verb tenses:

"he has decided to go hiking, so I went hiking as well."

But this doesn't:

"I went hiking because he has decided to go hiking."

Are there instances where present perfect (or continuous) + past simple or vice versa is allowed and correct?

EDIT:

This is a difficult concept for me to grasp. But thank you for all your help, because I'm learning a lot. There are many sentences that don't make sense to me. I came across this sentence today:

"[I think this is inferred: During those days,] We've watched the movie many times. I really wanted this life to continue."

I'm not sure what the present perfect is doing here. I don't think it is being used for describing what a person has done before, like "I've seen that movie before". Why is it not simply "We watched the movie many times. [It was so good that] I really wanted this life to continue"?