From one fabricator to another, Welcome to T-net!
Drum speed is a relative factor along with angle, length, feed rate, and spray gpm.
The purpose of a trommel is to wash all the gravels clean before they exit the drum as tailings.
You want to keep the material in the trommel tumbling long enough for the spray to wash the gravels completely clean. A steeper angle will cause the material to move faster through the drum allowing for a faster feed rate and less angle will keep the material in the drum longer requiring a slower feed rate.
If you are running the slurry into a gold cube the idea is to not overwhelm the capacity of the cube. You want the rpm's, angle, feed rate, and spray gpm's to match the capacity of the cube.
You will also want to maintain at least a 4:1 liquids to solids ratio entering the cube for best results. If the slurry is too thick you could loose gold. Better to run a thin slurry rather than too thick.
You stated that your motor runs 33 rpm. The drum speed will depend upon the gear ratio between the size pulley on the motor and the drum diameter at the drive. (that is unless it is direct drive)
Correct rpm's will depend upon the length of the drum from entry to slurry exit along with the type material being run. The shorter the drum the faster the rpm's will need to be. The harder the gravels are to wash clean the longer they need to stay in the drum (less angle)
If the rpm's are too fast a centrifugal effect takes place and the gravels wont tumble. Too slow with a steep angle and the gravels dont get rotated enough before exiting to break down the slurry into separate particles.
Hopefully this will help you to understand what you are trying to achieve and will put you on the right track to fine tuning your set up.
GG~