I have been pondering if BB only had reefers as main consist so I loaded a vcr tape (now that's a blast from the past!) of big boy from video rails.
in it it showed mixed consists of single dome tank cars cattle cars gondolas flat cars hoppers and even maintanence of way cars.
so if your short of reefer cars add all the mixed freight cars you care to it will be correct!
I even saw a flat car with a single wide mobile home trailer.
I am glad I decided to watch it as somehow nothing but a reefer consist is rather plain for my tastes.