This is a quick one because I can't find clarification in the rules:
If I give a monster with multiattack and a slam attack (like a doppelgänger) a nice pointy sword, does it still attack twice with the sword? Or once with the sword and once with the slam? What do the rules say?
A creature's stat block will indicate what kind of multiattack it gets to make.
For instance, if we look at a Black Bear (MM, p. 318), it says:
Multiattack. The bear makes two attacks: one with its bite and one with its claws.
If a bear were somehow to pick up a sword or two it wouldn't be able to use them for multiattack.
However, if we look at a Doppelganger, it says:
Multiattack. The doppelganger makes two melee attacks.
Sword attacks are melee attacks.
Thus, as per RAW, there should be nothing stopping a Doppelganger (or any other monster with such wording) from using its sword(s) and/or Slam attack for multiattack.
By RAW, monsters work exactly as dictated by their stat block. If their stat block does not have swords listed, then the monster doesn't use swords.
However, as a DM, you are also completely free to add, remove, or modify any or all features of a monster, or even come up with a completely new one as you wish. If you want to take an Animated Armor and hand it two longswords to attack with, that's completely your prerogative.
However, you should keep in mind that any change to a monster's stats will affect how challenging it is, and as such may require a change in the monster's Challenge Rating and/or Experience reward. Animated armor that deals 1d8 damage per attack is more dangerous than one that deals 1d6 damage per attack.
