Is being competent in Spoken English directly linked to being good at teaching, if there are not linked what could be an ideal explanation?

No, it's not. In order to be a good teacher you need many skills. You need to have a good knowledge of didactics, methodology, grammar and you need to have excellent pedagogical skills. Simply said, you need to know how to teach, interact with students, you need to be able to see what they need and how to improve their skills.Teacher must be an actor, a performer, giver, receiver, listener, organizer etc. and being an excellent speaker is a big advantage, and i would say a must. So, all these things combined make an excellent teacher. :)

IT is not considered that a good speaker for English languge is a good teacher, it has many considrations, some native speakers aren't good teachers I touched that myself in one of International schools but students just acquire the correct pronunciation that is all but in using Grammer they aren't good at it and when we come to that point with them , we find they say that we want to be British more than the British themselves and we should concentrate on meanings only,so good teachers are not supposed to be native speakers , Indians are good English teachers the Egyptians too and other nationalities although about half of them aren't good speakers but they manage it well all they lack is the good pronunciation and they will be perfect, you can see them working in some international schools in Egypt and abroad.

The good teacher is a good conductor as metals , metals expand and shrink so the good teacher is like that , we have differences , a teacher can expand and has no limit with good students never stop as long as they are good and are able to get more and more , he also shrinks when students are not good in understanding here a teacher must use small teaching units , when students finish a unit he moves to another whatever it takes from time but all what he should care is the good out put. thanks a lot.

you said spoken english?what about writing and other skills. i don't think it  is suffecient to be a good teacher.

teaching skills are much more than speaking a language,it is the art of teaching.

he or she must posses teaching skills

Never. A goos pseaker may not become a teacher at all. Methods, knowledge of didactics and grammar, pedagogical skills are very important, too