I use regular henna. Usually I henna on dry hair overnight. Rinse well, co-wash, DC.
I have washed first, then henna, the rest the same steps.
If I do an oil rinse, I throw it in after rinsing henna and before co-wash.
So looking at your reggie, I would not pre-poo, then shampoo, henna, oil rinse, co-wash (or vice versa - i think the oil helps the grit slide out easier for me) and follow the rest of the reggie.
ETA: Not sure if cassia works better on wet or dry hair. With regular henna, I get more dye deposit on dry hair.
ETA 2: or like the other poster said, henna first then follow with reggie. If cassia can be as drying as reg. henna, maybe skip the poo, but go by how your hair feels, of course.