I think it really depends on the crossover and to me Smallville and LnC just don't mesh very well.
Stargate I can actually see working. There's nothing in LnC that says the Stargate program can't exist and the the SGC could certainly have someone watching Metropolis and her favorite alien. (In fact, I signed up to do that one and never got to it. Maybe over Xmas.)
I'm afraid a Smallville/LnC xover would turn out like my 'Plane Storm' or some variation.
One set gets transported to the other universe, LnC see the other pair as childish and immature, try to fix things, fail (because the SV pair are childish and immature).
I can see this conversation:
Lnc (3rd or 4th season) Lois (Lois 1) to Clark 1 after meeting SV Lois & Clark and watching them a while.
Lois 1 : Please tell me I wasn't that psychotic at her age?
Clark 1: (has the good sense not to answer directly) She IS very driven and really should cut down on the caffeine.
Lois 1: And what's with him? Shouldn't he be traveling or something?
Clark 1: Well, that's what I was doing at his age. Obviously things are different here.
Lois 1: They cannot be that different. I mean he's... You were never...
Clark 1: (shrugs) He's obviously had a much tougher life than I had.
Lois 1: (muttering) Tough my...
Clark 1: (hearing something) I'll be right back.
Lois 1: Clark!
Clark 1: Maybe all he needs is an example?
