As far a playing the bass side of a cajun box: I use my index finger and little finger for the bass keys and I press the air button with the heel of my hand, not my thumb.
Rather than an Ariette, get a Hohner model 114, especially those made in Germany. The 114 is much better than the Ariette and can be "just" tuned to give it a real cajun sound..Don't get a Gabanelli "Cajun King". Better off with the Hohner 114 until you can get the "real thing" made by one of the several builders in Louisana and south east Texas.
Jude Moreau and Ed Poullard in Texas make very nice boxes at a good deal lower price than builders like Marc Savoy (Acadian). Savoy's boxes are fabulous if you can afford one or find one used. Junior Martin (Martin) is perhaps the builder with the next best known reputation other than Larry Miller, whose boxes are now made under his supervision by his son. Over all, you can't go wrong with a hand made Louisiana box from any of a dozen or more builders.