Concerns about TAGGING on STEEMIT

I have started posting on steemit for the past 5-6 days and have been discovering a lot of rules/etiquettes and dos and donts, upon interacting with the community. I would wish to post some questions, anyone is free to answer. 

1) What tags/categories are relevant/irrelevant or considered good/bad as per the community?

I came across a post by @berniesanders, who seems to be someone very reputed in the community - warning about using irrelevant hashtags/categories to promote their posts.  Heres a link to the post

I wanted to ask these questions in the comment section there, but noticed that there was no "Reply" button. Is it because I am a newcomer or I need to be followed by the person to be able to reply? I was not sure, so posting this as a separate post altogether. 

2) What is the logic to be followed for tagging/categories on steemit? 

Example - please see this post I made couple of days back - Selective coloring photography - Butterfly (EXPLAINED) . 

At the time of finalizing the post, in the column for adding upto 5 tags - I added the following tags "photography", "photoshop", "butterfly", "insects". As per my understanding, each of these 4 tags are relevant to the post. However something such as "insects", I clicked on it and discovered that there were no prior posts in that category, which means by tagging "insects", I may have just created a new category. Reading the post by @berniesanders , I did not know if tags such as "insects", "Butterfly" etc will be considered as creating new categories unnecessarily and will I run a risk of being flagged/downvoted? 

3) Shouldn't categories and tagging be separate from each other? Tagging should be done keeping only SEO in mind? Please let me know if I am right/wrong. 

Tagging should be done with keywords that are relevant to the post, so that the post can be indexed easily by search engines and relevantly displayed by search engines depending on the keywords searched by the users. Tomorrow if and when steemit becomes a really large social media platform and someone comes to the platform searching for photos, videos, posts about insect photography - should my post not be indexable for such a search query? And therefore shouldn't "categories" be something totally separate from "tagging"? In my humble opinion, users should be posting under pre-selected list of categories with whatever tags they consider relevant or irrelevant. Should this not be the ideal case? So that ever post does not go on creating new "categories"?

4) Just a suggestion

It would be good to have a direct link on the top of the website with something like "best practices" or "guidelines" or something, that guides the newcomers on all these basic questions. Or maybe a category called "steemitbeginners" ? A lot of what a person like me is discovering is merely by chance. In the steemit chat, I was shared this post by @beanz which I found very useful - Unwritten rules of Steemit

Please feel free to answer or respond to any/all of my questions/suggestions above. Posting this publicly as I think many newcomers like myself can benefit from it.