Hello,
It is unlikely that either of those regulators could handle the combined load of 18 servos. We typically recommend a budget of 1A per servo for standard servos; high torque servos can be many times that. I suggest looking up the stall current for each of your servos and adding the stall currents together to get the combined current load for your system when all servos are running.
Please note that the Mini Maestro servo rails can be segregated into banks of six servo. Each bank of six servos can only handle about 6A, and each header pin can only handle about 3A. If your load exceeds those ratings, you should probably power your servos away for the Maestro board. You can read more about separating your servos into banks in this forum thread. You can see additional pictures showing the cuttable trace locations in the “Mini Maestro Pinout and Components” section of the “Pololu Maestro Servo Controller User’s Guide”. If you power your servos away from the Maestro board, you should be sure you have a common ground between your servos and Maestro.
-Derrill