All of the sentences are grammatically OK. Consistency of tense is more about good style than proper grammar. However, be aware that it's also good style to omit repetitive words when they are not necessary, as in your first sentence:
He should not repeat ... but instead (he should) focus on ...
The second "he should" is not necessary, since the reader already understands that the writer is offering some kind of advice. So these should both be read as the same tense.
The other sentences are not as well written, but still natural English. More parallel comparisons could be written:
He doesn't like watching movies but instead (he likes) reading books.
We didn't want to harm the person, but instead (we wanted to) help him.
Unfortunately, good English style is difficult to teach. The best way to learn is to read a lot, and emulate the style of good English writers. Most newspapers and other large periodicals have good writing style, but even this varies between something like The New York Times or The Wall Street Journal, and some smaller newspaper, or between Time Magazine and People Magazine.
[Edit] As virolino mentions in the comments below, repetition is not always bad. It's fine when it serves a purpose, such as to emphasize or clarify. You have to judge from the context whether it adds to or detracts from the intended meaning.