The apothecary would be an outside the guild contact. As for the separate fields of assassinations they are all part of what the guild does. You train in the art that suits you best. Good with a bow, then train in accurately hitting you target. Good at slipping around undetected, look into dagger and poison uses.

While specialist are common in the guild. Over specializing is not advised. It's advised you have at least a teir 1 understanding of all the methods. So you can easily make back up plans if your main method fails.