An easy fix for the Saibamen is to a take a page from the various DBZ video games. One of the DBZ games is an RPG on the Super Nintendo and Saibamen are of course a random enemy encounter you get early on. But if I recall correctly that game used recolors later on to indicate a Saibamen as being higher level than earlier encounters.

What if we did the same thing? Raditz uses the lowest quality Saibamen and then Nappa plants his own batch later and then the heroes are surprised when these different color Saibamen turn out to be more of a challenge than the batch Raditz used?