( I am not a legal proffessional )
IMHO, the law of the land is supreme.
Any person in the armed forces comes under two laws - Military law and the Civil law. He can be tried by Military law as well as the civil/criminal law if the offense so warrants.
You can look up the Manual of Military Law ( a.k.a MIML ). All Army Officers and Army units will have a copy of this book. There are two volumes which cover the SCM/GCM procedures and acts.
You can also consult the JAG brach. They will be very helpful and will not charge any money for consultation. There are retired JAG officers ( Military Advocates) who may be able to assist on fee basis.
If a soldier commits murder/rape, a GCM is automatic , ie Unit Commanding Officer will instantly order a GCM. Until GCM proceedings are not complete, he will be kept under arrest. There is a Jail inside every unit.
If I may ask, are you representing the accused or prosecuting ?