In case anyone sees this more recently, there is now a newer class called Handheld Computing Device (cmdb_ci_handheld_computing) extended from Computer (cmdb_ci_computer), with the addition of mobile-relevant fields such as Phone Number, IMED, MEID, ICCID, etc. Additionally, as @Ben Sweetser has recommended multiple times, you can add Form Factor to the individual Hardware Model records to designate Tablets VS Laptops VS Desktops, and then set the Model Category appropriately.
