Are there new monsters in the DLC?
I don't think so. It's more like you can meet "rare" mobs (and supposedly rare drops), but I haven't downloaded it yet.
Can this be completed in single player ? Are there AI bots to use as missing partners ?
For campaign mode you can take NPCs with you, and they're quite smart.
Due to how story progresses there are moments you're forced to go mission solo.
And your skill setup would differ quite differently when you have to do that.
(You'll learn it as you progress).
For multiplayer mode (like Avalon missions on SS or gathering hall on MH), you can go solo, with your buddies, NPC or bots of buddies you have played with.