[Edi]

The management of wandering independents is basically done so that you need to recruit cheap commanders like captains or goblin chieftains or similar, give them some troops (10 spearmen/swordsmen and 5-10 archers) and put them on snake patrol to kill anything that wanders around.

As for why any wandering stack is generally "snakes", is because they often are (swamps can randomly spawn snakes and serpents sometimes) and because the wandering stacks sneak up on you and snake all the stuff you spent so much time and blood (and consequently treasure, troops don't come cheap) conquering.