Do these gas beam mount engine's require an aluminum beam mount or is a composite beam mount ok?

Whichever way you want to do it. Plastic composite material, metal, hard wood or a combination. It all works. Some people prefer metal as it may have less vibration effects due to the metal being more stiff. But a carbon fiber beam mount would help lessen vibration harmonics even more since it tends to behave differently and deaden such harmonic vibrations.

With my two DLE 20cc I have had great results with the DurBro Vibration Reducing Motor Mount.

Some guys are recommending the Great Planes 120 size composite mount for the 20cc engines. For my RCGF 20, I used the composite mount that came with my 80" Seagull Decathlon to see what happens? It's got a couple dozen flights on it with no issues yet!

I am using the Hangar 9 120 size composite mount same one as in the pulse 125 on my Funtana 125 and it is very low vibration and holding up well. Be ware of some of the cheaper white plastic or black hard composite mounts that tend to be brittle and have cracked on a few guys.