Any weapon can be mix/combo as a sub class.
Sub class weapon choice is just a bonus to your current stats not necessarily wanting a combo skill or not. Tbh no one uses the combo skills anyway; 2min cd, low dmg.
For example a Ravager focusing on crit stat would want a sub weapon with high crit rate/dmg(staff, cannon etc). While Classes like Duelist might want to have a support sub class like Bard, it all depends on what your building towards.
Just keep in mind that sub class are just bonus stats. Many ppl do tend to focus too much on their individual skills which is rarely used(except for heals, gap-closer etc since sub class skill have way longer cd than main class).