Mixing wattage is fine ... but remember that in a 2x12 each speaker will need to be able to handle 1/2 the amps wattage (1/4 in the case of a 4x12) ... so a Retro 30 (75watt) and a Reaper (30-watt) would only be appropriate for an amp of up to 60-watts RMS. As the weaker link, the Reaper would blow long before the Ret.
A Reaper HP (50 watts) & a Ret would be good for up to 100 watts RMS.