How much room for expansion

This would be excellent for this purpose. Yes, it will set up in plastic containers. I have not witnessed any significant expansion during curing. Leave a small amount of room at the top of your planters (1/2" maybe?) and I think you'd be fine.

In general, concrete shrinks as it hydrates, so you shouldn’t have to worry about expansion of the QUIKRETE Fast Setting Concrete Mix. Since you are not using the concrete for setting a post in the ground, the concrete should be mixed with water before placing it in the planter, as it will not be absorbing any water from the ground. The concrete will set inside plastic; however, when wet, concrete is very alkaline material, so you may want to verify that the plastic the planters are made of is somewhat resistant to alkaline material, as PVC is. However, the alkalinity will continue to decrease over time, so if the planter holds up past the first 28 days, it will tentatively last well beyond that.

Concrete by its design seems to shrink as it absorbs the water you add. It fills in voids and air gaps. Just remember that a 50 lb bag will still weigh 50 lbs when it has set and dried. I would use a wooden stick to tamp the concrete into the plastic planter boxes to make sure all voids and air pockets are filled. Some day the plastic boxes may crack and break away from the concrete. You could then paint or stain the concrete if this happens and the concrete looked good.

Don't worry about your planters and expansion, as concrete hydrates (cures) it actually shrinks a little bit. Once you start the chemical reaction by combining the mix with water it's going to set...whether it is in a plastic planter, under water or in wood / metal forms. I would mix the concrete in a wheelbarrow and place it into the planter rather than trying to mix it in the planter. Concrete is heavy! Are the planter boxes stout enough to hold the mix?