If you want to befriend them in order to acquire them as vassals, you´re too late. If they haven´t got a single holding left they´re close to extinctin. Let them perish, the darned bastards.
If you fit the positive relation required before you capture a prisoner (note the -10 relation penalty for taking him prisoner) you´ll be able to recruit him after battle if you talk to him again - you´ll get a ransom option and if your relation´s good enough the recruit option.
If they´re zapping around the map with these tiny, tiny armies, it´s usually best to hunt them down with a less than 20 men cav army. Check all small minor parties as well, the last lord may have been taken prisoner by some bandit faction. Or simply wait for them to perish by themselves.
Promoting vassals as companions is a bit problematic. Lords may become upset about them if they aren´t of noble origin.
Generally, you can´t take troops off your companions. You can only give them units. Any unit given to them in a town/castle will be added to the garrison (methinks).
Which kind of troops a promoted lord gathers depends on his personal background faction and can vary greatly from NPC to NPC. This part of PoP works just as native, so once you know which NPC replaced whom from native you´ll be able to realise which troops he´ll pick up upon promotion once you know which native faction equals which PoP faction
Generally, it´s disadvisable to use companions as lords, especially if they´re ones you´d better use yourself. If it´s a totally worthless one (you never use him) it doesn´t matter. If it´s your best buddy it´s problematic because if you piss him off enough he´ll either leave your faction and desert you (means you´ll have to fight the bugger if you war his new faction) or if he goes exile, he´ll leave Pendor forever.