the Sstram and the Mak may have been very useful throughout the empire however there can be a number of reasons y they would not be on earth
1.) Bodies may be entirely i'll adapted for "shock" troops or as workers
2.) Considering the fact that Visser 3 is near the top of the military command, and is used to fighting battles all the time, his army he would focus on earth would most likely reflect his personal wartime ideal's in which the taxxons (pilots of wartime machines) and Hork Bajir (Intimidation and full use of limbs). Since these two species are most capable of fighting a war i doubt that Visser 3 would add any new species especially when he's constantly on the verge of getting an all out war approved on earth.
3.) In one of the books (i forget which) Ax says that there are several species fighting the Yeerks but that only the Andalites can stop them, This means that towards the rise of the Yeerk empire, these species could have sided with the Andalites to stop the Yeerks and were obliterated
4.) Since the Vissers rule the empire with their own private armies more or less, a single Visser might just reign over the entire species in some far flung part of the galaxy.
These are just my humble opinions feel free to point out any errors or add criticism =D
