One background commentary I have suggests that the size of the army is fairly substantial:
Here we discover that Abram has a household of significant size (318 recruits or retainers). The word used to describe these men occurs nowhere else in the Old Testament, but does occur in an Akkadian letter of the fifteenth century B.C. Whether Abram is placed within the early part of the Middle Bronze Age, when the area was predominantly occupied by herdsmen and villagers, or within the later Middle Bronze Age when there were more fortified settlements, this army would have been a match for any other armed force in the region. Even as late as the Amarna period the armies of any particular city state would not have been much larger.
Matthews, V. H., Chavalas, M. W., & Walton, J. H. (2000). The IVP Bible background commentary: Old Testament (electronic ed.) (Ge 14:16). Downers Grove, IL: InterVarsity Press.
The New American Commentary agrees: "Abram’s leading (“called out”) as many as 318 men, all born to his own household, shows the patriarch’s substantial wealth and power."1 However, it doesn't seem so cut and dry, as seen in Kidner's comment: "Abram’s success with so small a company is viewed sceptically by, e.g. von Rad, who seems to overlook not only Abram’s allies..."2 That said, Kidner's volume is somewhat older (and so von Rad), so I would tend to trust the other two volumes above which likely benefit from newer research in archaeology, etc...
Further, I'm inclined to think that the text is stressing his power since it seems to stress in so many other ways Avram's wealth and power as a result of God's favor (cf. 13:2). This passage in particular demonstrates the kingliness of Avram, which is often done in the Tanakh by stressing the wealth, armies, and victories of a king. It would seem to fit then, that the mention of 318 trained men is meant to show that Avram has a sizable army even in his own household.
As far as the second question about the night ambush, the same background commentary mentions, "Abram uses the strategy of nighttime ambush, which is attested in texts as early as the Judges period in Egyptian as well as in Hittite documents. This was similarly attested in the Baker Encyclopedia3:
In one of his documents the Hittite king Mursilis speaks of the trouble he encountered from a small army of irregulars who used the night ambush to disrupt his advance: “They did not dare to attack me in the daylight, and preferred to fall upon me during the night. [They said:] In the night we will attack him.”
There are other night battles in the Tanakh. While not quite a night battle, Joshua's army famously marches all night to Gilgal in order to surprise the camp there. Jon Ericson mentions above that Gideon's attack in Judges 7 occurs at night. And in 1 Samuel 14:36, Saul suggests a night attack on the Philistines. Obviously these accounts if historically true all occur well after the account of Avram, but I think it demonstrates that it wasn't highly unusual and nothing in any of the narratives suggests that these were dishonorable attacks that run afoul of the customs of the day.
1 Mathews, K. A. (2005). Vol. 1B: Genesis 11:27–50:26. The New American Commentary (147). Nashville: Broadman & Holman Publishers.
2 Kidner, D. (1967). Vol. 1: Genesis: An Introduction and Commentary. Tyndale Old Testament Commentaries (131). Downers Grove, IL: InterVarsity Press.
3 Elwell, W. A., & Beitzel, B. J. (1988). Baker encyclopedia of the Bible (191). Grand Rapids, MI: Baker Book House.