I dont get why they wanna add Welling but change the JLA cast. Unlike Superman Returns and Smallville co-exisiting at the same time, this actually is confusing.
I don't know about you, but Alan Ritchson doesn't exactly have the commanding 'Lord of Atlantis' Aquaman characterization...
Not only that, but I doubt they'd want to use Kyle Gallner as the flash, considering he's more of a Kid Flash/Impulse character, where they'd likely be using Wally West or Barry Allen, instead of Bart.
As for Cyborg, he's a Titan, not a member of the Justice League.
Now it may seem like i'm ranting, and don't have an actual point. However, I assure you there's one there. Basically, the only character's you could use from Smallville, to fit into a JL based Supes, would be Lois, Lex, and maybe Ollie. Now if this is a 'Major Seven' type movie, then you probably wont even get a Green Arrow. So Justin would be out too.
Creative writing, and a lack of seeing Clark's day to day life could easily make up for not seeing the other characters, while giving the allusion that it 'could' be a movie based off of smallville, or it 'could' be in the continuity of the S:TM/SR movies.
Here's the important part.
Welling's Clark Kent is the only character you need to link something to Smallville.