Is it OK to rely on an ADC's internal protection diodes?
The data sheet should cover this. Usually, if there are internal diodes specified they will be adequate for protection. 

Note that diodes, internal or external, are intended to prevent IC destruction BUT not to prevent malfunction in normal operation if they conduct. Voltages should always be such during measurement that essentially no diode protection current flows. Keeping Vin inside the supply rails achieves this. Slightly outside MAY be OK, but with ADC especially, no guarantees.