Originally Posted by dizzie:
“He would actually be a fantastic choice, and really, the right age range to play the older cousin - plus, they've left the Lois and most of the other Superman characters untouched so far in the Supergirl universe. Only Jimmy/James has been a significant 'recasted' and reimagined character, that diverts massively from the Smallville 'verse (and, yes, there was a version of Supergirl in that show, but hey ho!). I suspect that could be forgiven if they managed to incorporate a few good stories, touching on both CW shows in a clever way. After all, Smallville managed to co-exist with the Superman Returns reboot, so there's already ample demonstration that two versions of the same character can work in different medium.”
Given that The Flash crossover implied that there are different Earth versions of shows - indeed it is a big part of The Flash altogether that there are multiple Earths with similarities and the same characters but varying in significant ways - then this is easy to overcome.
If they are upping the interaction between all the DC shows then Supergirl will doubtless meet Oliver Queen and the rest which would create even more of a mismatch with Smallville as those characters were in the later series of that show too. With similarities but also big differences.
So no reason it could not be done at all given this existing key story element of the DC universe shows.