Hi Jillybean-
Obviously, I am not Cathe either, but if you were looking to add a body part to your workout after doing kickboxing, I would suggest adding leg work.
Maybe doing just the first portion of KP and C and then adding the first part of PLB or the first part of the PS for Legs would allow you to get that strength workout of a body part in.
Because those two strength workouts are so squat and lunge oriented, it would be a nice complement to all of the various kicks in Cathe's kicboxing tapes.
I like to do the CTX kickboxing cardio on the days I work legs if I want to do cardio that day. Because the kickboxing isn't as leg intensive as step or running, it seems to work nicely. However, IMHO, doing all of Cardio Kicks or KP and C and THEN tacking on lower body work would be way too much.
Take Care, Lynn