I had always assumed that a contingent refers to more than one person; after all the dictionary definition of the word states "a share, quota, or group, especially of soldiers." Additionally, I would think that if Beall was referring to only one person (namely, lieutenant Beckett Howell), he would have just stated, in his report, that on the CSS ALABAMA there was one Marine officer, instead of using the word "contingent." I have a feeling that some of the sailors on the ALABAMA were in fact employed, perhaps on a temporary basis, as Marine guards, as every large vessel needed a disciplinary force to subdue those who were incalcitrant, but these members still maintained their original ranks aboard the cruiser.
I have copies of most, if not all, of the first hand accounts of personnel from the vessel, and often, on re-reading these accounts, one will note passages that include statements that may seem of a trivial nature, yet these trivial statements reveal a lot of new data. For instance, Semmes himself, in his volume on the vessel, notes the names of two crew members which are not shown anywhere else, or in any muster roll of the ALABAMA. I am sure Semmes would not have easily forgotten the names of some of his men, especially those who made an impression on him. Furthermore, his account was written within four or five years of the end of the war, when his memory was still fresh with the events of those stirring days. What I am convinced of, is that the statement of Beall was probably made for the logical reason that there was more than one person on the ALABAMA who served in the Marine "contingent." Since many of the records of the CSN that were held in Richmond were destroyed at the close of the war, I guess we will never ever know a lot about the personnel of this particular service.