What Ho! I.R. Pirate.
If one were to be really pedantic which, in the context of world wide CAAs and aviation law, is probably not a bad thing at all, one would have to say that, under certain circumstances, a commercial licence holder could act as Captain on a Beech 1900. However, a commercial licence holder could not, one hazards, act as Commander on a Beech 1900. There is a subtle difference between the two roles which has been neatly, shall one say, explored, by a certain international Nigerian operator of small sophisticated German jets.