With 2 Sikhs headed to Marine boot camp, legal battle is ongoing

Look at Great Britain in WWII, a multi nation Force having manifold distinct Armies and even more appearance/ uniform differences all operating smoothly. The Regalia of uniforms/ the appearance of and deportment of those serving, the officers and such , the distinct and singular purpose of any soldier in any Army / the chain of Command, all worked well and little problems were had in dealing with the enemy effectively.
That said, The US is a couple of Decades behind the modern Times when it comes to what is and isn't allowed uniform-wise.
If the person with the differences, is able-bodied, person with the uniform issue, if it does not compromise force integrity DoN't Make it an issue.
America has hundreds of nationalities/ Religious/ personal/ traditions ... why deny these persons from serving over a Turban/ beard/ etc., . As long as their in cammies and fit to serve i can see little else being any problem. If it is something like uniform regalia / headdress difference etc., It can be dealt with by introduction/ expansion of / amending the uniform regs and including the additional military/ Social/ Religion when it does not interfere with combat readiness kept appearance of the Soldier.
The rest is just simple judgment of the CO .