If we using the old "watt per gallon" to calculate, you should be need at least 6 watt per gallon. However, in a small tank as 20G, you might reconsider the combination. Lighting is not the only thing you should consider, you also need to think about the water flow. SPS
need very strong flow but soft coral hate strong flow. If you are in a bigger tank you can do it simply choose to place coral at different location. However with smaller tank, you will not have that benefit. Therefore, you might have to choose either or.