Please merge "solusolution" to "chikahii". This is the same artist.

For your first question, the closest I can think of the tag you're looking for is real_life_insert. At the very least I think it applies to this post. The the second, I don't think such a tag exists.
EDIT: I'm having second thoughts, looking at the wiki, it's probably not the right tag for this. My bad

Try looking at reference, reference_photo, reference_work, derivative_work, reference_inset

If we were talking animation I'd guess rotoscope (rotoscoped?)

Hi! I can see tag 'ebike' among the artists on this picture: gelbooru.com/index.php?page=post&s=view&id=9797490
Is it really an artist? If so, I think, we should introduce a separate tag for him, like 'ebike_(artist)' to distinguish him from electric bicycles on pictures. Shouldn't we?

That doesn't even look like an electric bicycle.

If we were going to have a tag for electric bicycles, it would be "electric_bicycle". There would be no collision.

Please merge the following characters and add original to them:

Mattie_(ego_trigger), mattie_(matsu_sensei) and mattie_(ego-trigger)

Matt_(ego_trigger) and matt_(matsu-sensei)

Lass_(ego_trigger) and lass_(matsu-sensei)

raine_(ego_trigger) and raine_(matsu-sensei)

otome_(ego_trigger) and otome_(matsu-sensei)

pyre_(ego_trigger) and pyre_(matsu-sensei)

I know there was a bit of a debate between the Oppai_loli and shortstack tags. Before I tag this artist (gelbooru.com/index.php?pa...list&tags=mato_tsuyoi), which side does their art fall on? I know Flubberbutt edits alot of those kinds of posts.

I personally feel it falls more on the "shortstack" side.

i'd say oppai loli simply because all of them lack the hips and thick thighs