The only instructions regarding specifically the soldiers life in the NT is a Prohibition from extortion, providing false witness, and not to complain abou their pay. That's about it. I don't see what you have to teach on it futher than that.
(1) Thinkingstuff here raised an important question as to our first example where soldiers are mentioned in the New Testament.
Are these really the only instructions specifically regarding soldiers? Perhaps they are, we shall see shortly.
But a better question would be,
Are these the only instructions which apply to soldiers?
Here I think we'd have to say no.
The first observation here, is that these few instructions aren't really even properly representative or exhaustive of what John the Baptist must have taught his listeners, soldiers included.
This cannot represent all that soldiers are responsible for, anymore than we could say that
"Take no more than what is appointed to you" is all that tax-collectors were responsible to do, or
"give one of your coats to him who has none" would be everything John the Baptist expected his other listeners to do.
It is obvious when we examine the context, that
these are only short examples of the type of teaching that John the Baptist preached, which Luke records for us, to give us a taste of what preceded Jesus' public ministry. (Luke 3:10-14).
Luke's Gospel is naturally not only about soldiers, or even primarily about soldiers. Nor is it even about John the Baptist. All we are going to get is a brief sketch of both: some samples to give us the picture and guide us. These sketches will be like stick drawings compared to the full story.
Soldiers are perfectly familiar with this. When you hand in a report, you don't write every little thing that happened, especially if nothing unexpected happened. You just say,
"patrolled perimeter, no encounters" or something brief. Luke similarly, gives a brief report giving the gist only.
It is reasonable to assume that John the Baptist intended the soldiers listening to understand that they also were expected to
"give an extra cloak to someone who had none", and
"not take more than their due" , just like other listeners.
That is, the words given to each group were not
exclusive to them,
nor did any specific instructions exempt them from other instructions given openly to the crowds and various parties.
The soldiers must have hung around to listen to other teachings from John the Baptist, equally applicable to them, as others did.
The soldiers weren't looking for loopholes, like lawyers, trying to minimize their responsibilities. They were honest men, seeking from John the Baptist the real goods. They wanted to know where they really stood, and what God expected of them.
They trusted John the Baptist to give them some guidance, and so can we.