ellipsis - Some clarifications are needed about the grammar of Elliptical Structure

As far as I know, one application of elliptical structure is the omission from a clause one or more elements which has been repeated in previous clauses. However, I do not know whether there exists some specific general rule stating how and when to apply this structure.

Let me be more specific:

1. 
   

   Is it correct to say that one can always omit any word which has been repeated before?

   
   


2. 
   

   Is this structure optional (I have read somewhere that sometimes it is obligatory, for example with cases of comparative deletion (for example, the sentence “More students were in the class today than were there yesterday”), but I also have read somewhere that the omitted words can be added without producing an ungrammatical structure)?

   
   

I want to emphasize that I want to know the answer to my question for formal English used in academic writing.

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Updated

After pondering the useful comments, I conclude that one should not consider any omission of words as ellipsis. In fact, ellipsis is the omission of elements which are understood in the context of the remaining elements and recoverable from the linguistic context. So examples such as “seeing more examples and gaining _more experience” and “making more effort and reading _more grammar textbooks” cannot be considered as elliptical structures since the second more is not recoverable from the linguistic context (We would have two sentences with different meaning before and after the omission of the second more).

So I think the first question should be rephrased as follows.

1’. Which repeated elements of a sentence can be omitted in order to get an elliptical structure?