Edit: In an attempt to not make a gigantic wall of text, I ended up cutting important information from the question. I have re-elaborated the question for better understanding of the setting. Sorry for the inconvenience.
In the year of 2704, humans have entered an intergalactic war against a species of elementalists (long story short, they are part of a group of humans that got separated hundreds of years ago, whose powers they themselves cannot explain how they achieved). After a long year of losses, they managed to create and mass-produce armor that could resist (but is not immune) to the elements and millions of sentient robots to help them in combat, balancing the odds.
In addition to this, two projects using genetic engineering where conceived. One, called the "War hound Initiative", with the objective to create tamable war animals. The other, called the "Post human Experiment", focused on making both soldiers and civilians physically and mentally stronger. While the former eventually became a failure, leaving mutant wild animals through the universe, the latter was widely approved and helped humans to have not just a fighting chance, but also a chance to win.
Seeing how improving humans gave good results, they decided to recycle the "War hound Initiative" and use animal DNA to continue their artificial evolution. This was the "Therian Initiative". Do note, however, that these "therians" still had look human enough to not be considered abominations. Therians based on dogs and cats still have human faces, for example.
Among some of these creations, we have: Ferals, based on various canines and felines to increase rearing, vision (especially at night) and smelling; Mimics, using the mimic octopus to create the ability to swin under water and to change the subject's skin to match the enviroment; Angels, with bird DNA, creating flying units without the need of a jetpack; And Giants, created from elephant and rhinoceros's genome and are very strong and durable.
Now, considering that they decided to create centaurs to help them out, what type of centaur would be the most combat efficient? Would the classic horse's speed and carrying capacity help create a support unit? Would wemics (upper body of a human, lower body of a lion) be better for stealth purposes and physical strength? Would it be something else entirely? Are centaurs even viable as combat units?
Here is a list of things to consider:
• The centaur will have to traverse many types of geographical and urban landscapes.
• There must be an distinct advantage to the centaur form over, say, just making human with horse/cat legs or creating a new vehicle.
• Most of the military budget when it comes to vehicles goes to developing spaceships. What is employed along ground forces can be taken down by infantry.
• How does centaurs fare against normal infantry, elementalists, giants, and other vehicles. Remember that they are using guns and that they can survive some bullet hits and elemental attacks.
• They cannot look downright monstrous. However, there is no limitation to what can be done, so long as the parameters are reached.
• Finally, disregard things such as "how would such a creature live in society?”. Because: 1) What is necessary right now is to make something capable of fighting; 2) Therians will eventually being fed up with being used like animals and ditch humanity for their lack of ethics while creating them.