To get to this type of comraderie, all of the recruits have to look the same, sound the same, march the same, talk, eat, sleep, crap and generally get robotized before they can rely on each other.
If the Sikhs want to go through that and AFTER getting assigned grow their beards back, and wear their tubans, I see no problem with it. Sikhs are fanatic friends and fearsome fighters. Any of the services should welcome them.
The flaw in your logic lies in the fact that Sikhs tend to be raised with the warrior perspective. Boot camp would not be diminished by the beard or turban of a Sikh. I’d be willing to bet goodly amounts of money that such Sikh’s would more than carry their weight, AND would be sought after platoon members.
If nothing else, think of the poor Drill Sargents - should they not have a few “maggots” who have been raised in a family tradition of warriors, instead of all too many of the usual ‘thugs with attitude’?