Currently, with bug 1691474, the position of a add new search engine is betwen existent search shortcuts and the local search shortcuts(bookmarks, tabs, history).

IMO, I think that the position of the add new search engine should be either at the end of the search shortcuts (including the local shortcuts) or maybe even positioned next to the search settings button (adding a mock), since in effect it's closer to a search setting rather than an active search shortcut and makes the button more intuitive.

There is no spec here, I put them there because they are search engines, thus I didn't want to group them with local search. Additionally in most cases (site providing 1 engine) clicking on it doesn't cause any disturbing movement, the + badge just goes away naturally.

I like your idea of moving them closer to the settings button, but then on addition the button would disappear from there, appear in engines, pushing away local buttons. All of that movement may be problematic to some users.