I know the DSM2 transmitters will operate the older DSM tech, but will the older DSM transmitter module still operate a DSM2 receiver?

From what I know, it'll depend on the TX whether its spec accepts DSM receivers. Just like the old DX3S is unable to accept DSM2 RX but the new one does. Just tht I don't see any point cause the RX won't operate in DSM2 speed.

I got a DSM2 receiver as part of a trade and was curious on compatibility with my Helios/Spektrum Pro Module.

Sorry but no. You need a DSM2 transmitter to work with a DSM2 receiver. The only radios currently compatible with the DSM2 receivers (SR3100 and SR3520) are the DX3R, DX3S (SPM3140 P/N) and DX2S.