Using a Ebike controller for a hobby BLDC
Can I just pick one of these up and use one of these with a SMPS?

Probably, provided you build the motor correctly.
Bear in mind that a SMPS generally can't handle current being forced back into it, which happens when the motor is being slowed down by regenerative braking. You may need to put an active clamp across the power supply to stop the voltage from rising.

will they work within a range inside of whats advertised (generally
speaking)?

Generally speaking a controller will work within its specified voltage range. If single voltages are specified they usually refer to the nominal battery voltage, eg. a 48 V Lithium-ion battery may consist of 13 cells of 3.7 V each nominal, and 4.2 V fully charged for a maximum voltage of 54.6 V. A controller designed for this battery would have to handle at least 54.6 V.