having the right genes helps, I feel that if anything, genes dictate potential fates and potential destinies (and yes, I am a big fan of Metal Gear Solid, if anyone caught the reference.) Clones and Clone commandos still have to be trained. They are flash trained in all the basic combat skills and specialities, and then they're REALLY trained, vigorously, thoughout their accelerated lives. You have an extremly effective and competent soldier in the end. A Spartan isn't that disimilar. The original 6 year old cantidates were selected from genetic markers, conscripted into the military and trained all of their lives up to the point where they received their armor. Sometime between they were biologically and cybernetically enhanced. then they went to work. Originally the Spartans were created to quell uprisings in the outer regions of human controlled space (halo universe here), then the Covenant Showed up and the mission changed. Spartans are around 30 - 40 years old at around the time of halo and halo 2, and have been in combat, REAL combat, not similated, since they were around 17 and 18 years old. Spartans have alot of experience to back them up.
A Spartan would know how to adapt to a changing combat environment. They had to, when the Covenant showed up. And Spartans are trained to fight in any environment on every kind of habitable planet where they would have to fight other humans or the Covenant. A Republic Commando is trained to do the same, and has an encyclopedic knowledge on how to fight hundreds of different species on hundreds of different worlds. Would that give the Commando a 1up on the Spartan?
Isn't the Spartan just a Human? A Wookie is not a human. How would knowing what the respiratory system of a trandoshan looks like, help in killing a Spartan?