As with the general consensus of the thread, mixing the grades will AVERAGE out your shrimp quality, not send it spiraling up or down.
Over time with enough culling the lower shrimp and higher shrimp will eventually result in the same-ish grade with what you started with, but you'll have more shrimp with a higher average quality. That's just how it always works.
I keep 2-3 shrimp species per tank, making sure to only mix the ones that live in median water parameters and will not interbreed. Hybrids are generally really ugly, most of them are brownish or less vibrantly colored, and the good ones don't breed true.