A certain planet, currently over 50% populated by humans, features a planet-wide civilization where almost every citizen is given military training. The planet has recently become a major trade hub for a galaxy containing upwards of 20,000 known races, many of whom are not remotely humanoid. Members of several thousand of these races have begun to settle on the planet in question, and in recent decades, they have begun to approach nearly half the population.
Coordinating fleet tactics was a chore in itself, but now this society is in the process of designing a military training program that will allow members of virtually any race to fight together in ground combat.
Psychologically, I'm assuming that these species are comparable enough to humans that cognitive processes will happen in roughly the same way for most of them, regardless of how much suspension of disbelief that requires.
Conjecture regarding the feasibility and infrastructure of this planetary state, linguistics issues, the dangers of inviting new citizens to serve in the military, or other challenges with the scenario are not of interest for this question. This is about developing a basic training program for physical combat.
My thoughts so far (feel free to critique these):
A) Any mobile species would likely have a support structure of some kind (like our skeletal system) and a system allowing for mobility (like our muscular system). Fundamental martial arts principles could be used to help each individual develop a custom martial art. To oversimplify, strike with the hardest, most stable parts of your body that also have the most muscle support.
B) Firearms could be developed using identical components, with attachable, customized grips and triggers designed by members of each species.
C) It may not be safe to assume that every species has a cellular structure, or at least one that easily repairs itself through mitosis. Some species may not be able to get stronger or develop endurance by pushing themselves.
The Question: What fundamental principles of combat would allow virtually any species of any build to work together with other species as an effective combat team? What physical and strategic skills could be applied to develop all of my soldiers-in-training into a formidable fighting force?