I want to use the SMU CMU Unify Unit (SCUU) with a B1500A. Are there any restrictions on the SMU configuration?

The B1500A SMUs are installed in a fixed order starting at the bottom and working up towards the top. The installation order is as follows:

HPSMUs
HRSMUs
MPSMUs

The MFCMU is always installed above the SMUs. The SCUU can only connect to HRSMUs and MPSMUs, which means that you need at least two of either the HRSMUs or the MPSMUs in order to use the SCUU (you can use one of each type). For example, if you install two HRSMUs, two MPSMUs and an MFCMU in a B1500A then you have to use the MPSMUs with the MFCMU, since the MPSMUs will be installed above the HRSMUs.