I have played the demo a couple of times now, and having 4-5 Captain's and the rest of the soldiers being Sergeant's after 10 mission doesn't feel quite right.
Therefore I propose a rank progression similar to this, keeping in mind that a Captain probably is the highest ranking officer of an elite special forces army that would be out in the field with the soldiers:
Private- soldier without training or experience
Private First Class/Lance Corporal- soldier with training or soldier without training but with 5 missions as Pvt
Corporal- soldier with 5 missions as PFC/LCpl
Sergeant- soldier with 5 missions as Cpl
Staff Sergeant- soldier with 5 missions as Sgt, there can be only 20 SSgt's
Gunnery Sergeant/Master Sergeant- soldier with 5 missions as SSgt, there can be only 10 GySgt's/MSgt's
Sergeant Major/Warrant Officer- soldier with 10 missions as GySgt/MSgt,there can be only 5 SgtMaj's/WO's
Lieutenant- soldier with 10 missions as SgtMaj/WO, there can be only 2 Lt's
Captain- soldier with 20 missions as Lt, there can be only 1 Cpt
If there are more than one soldier up for promotion but only 1 spot the one with most kills/medals/experience should get promoted.
Exceptional soldiers(kills/medals/experience) should of course have a chance to be promoted before they have reached the required number of missions for the next rank.
And for the starting 12 soldiers I think that this would be a good blend:
1 SGT, 3 CPL, 4 PFC/LCpl, 4 Pvt