Will any 3 cell lipo battery work for the B400 as long as the specs are the same (11.1V 1800mAh)?
Or can I use a 11.1V 2200mAh?
And does it have to be 20C?

Any 3S 11.1V battery will work.
mAh is capacity so the 2200mAh will give you longer flight times.
The only thing to be concerned with is as the capacity goes up the battery is usually bigger and heavier.
Many people are using the 2000-2200mAh batteries in the B400. Just watch the weight and dimensions.
The bigger batteries are a tight squeeze under the canopy.

Ahh but that's what I have a trex 450 canopy for

The nose area has much more space than the B400 canopy.

Yup, others are doing that too.
I wouldn't go bigger than 2200mAh because of weight.

I would stick with a 20C rating, regardless which mAh battery you get. You can look at it like horse power in a car. A 10C battery may not even get the heli off the ground.