Can you pump the tank through the cleanout?

No. Even if a hose could fit down the cleanout, this still would not provide a thorough cleaning. The tank should be pumped from the actual access lid(s) on the tank, which are usually buried 6-12 inches below the surface of the ground. Depending on the age of your system, the tank may have a dividing wall, in which case we will need to gain access to both compartments to thoroughly clean the tank.