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Each incoming link that is non-reciprocal will count as a vote for your website, whether relevent or not.
Yes when getting links, make sure you mix and match the anchor text or else you may get a penalty as the linking will look unnatural.
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and the links should be going to the homepage and links within your site too, to look natural to the search engines.
Another question: is dangerous having only related backlinks?
Not sure what you mean here?
The basic idea is that you want a link profile that looks natural. An example of an un-natural profile would be nothing but directory links all with exactly the same anchor text. But a natural profile would include a mixture of many different types and anchor texts.