Do monsters have unlimited ammunition, or do you need to keep track of each and every minion?
Similary, do they have to spend a minor action to stow/draw their weapons?
Monster equipment is really iffy. I generally just assume that they have whatever they need, because:
A number of ostensibly weapon powers don't actually require weapons, as in have the Weapon keyword. Sometimes I'll read an attack called Shortsword that has no keywords at all, and therefore RAW means that they require nothing to make the attack.
Many 'Monsters' that would use equipment don't have equipment listed. This often includes monsters that actually have the Weapon keyword on their powers.
Because of these, I just assume that creatures have what they need on hand, when they want it. However, while the majority of creatures that have ranged and melee weapons don't have interesting minor action options, they wouldn't be able to make opportunity attacks while holding a bow (usually), so I'd just track whether or not they made a melee or ranged attack, without having to declare minor action uses each turn.
As to ammunition, a quick look has me see that most creatures that have listed ammunition carry 30 arrows/bolts/whatever, which is more than anyone but the most ridiculous of Rangers would use in a single encounter. So, they have as much as they like. (unless you're running a protracted siege or something, I guess?)