When you posted about this on the ARROW board, I pointed out that he'd likely have nothing to do with the Supergirl series, since this new version of Supergirl has a very different history from the one we got to know on SMALLVILLE.
If Welling does appear as Superman and he IS in any way connected to the Supergirl show, then he'd have to be a Superman that's SIMILAR to the SMALLVILLE version, but not the same one. The two different Karas prove that.
That would be true, or else they could have Dean cain appear as E-2 Superman to Tom W E1 Version!
So could be 2 different earths that Super family live on, and the Flash/Arrow reside on, but have someone cause a crossover event...
Either way, barry would have to deal with then being merely 4th fastest person on earth!