Old habits use 3/4" studs for rolled section composite design. Are smaller diameter studs ever used with CJ sections to accommodate chord material thickness?

Smaller diameter welded studs (1/2″ and 5/8″) can be used. The welded stud diameter is based on the forces in the top chord as well as the top chord angle size. Table 4.4-1 and Section 4.5.4 of SJI Composite Joist specifications ANSI SJI 200-2015 contain the design requirement for welded studs.