I'm fitting a wall hung toilet using a Grohe frame and would appreciate advice on connecting the waste to the soil stack (to the right in the image). I am planning to connect the Grohe supplied waste (black pipe) to the stack using solvent weld pipes (don't trust the push fit not to leak) - the black pipe would fit into a 135 deg bend which will connect with a horizontal pipe to a 90 deg bend that will go into the push fit opening in the stack to the right. Will this be too rigid to allow fitting to the frame? Also, I was planning to run the water pipes from the floor behind the waste pipe - is that poor practice, would it be better to route them around the frame?