Having not played Land 4 yet I only have access to the MarioWiki article and it doesn't explain any significant connection. If someone does know on here though I'd love to hear the explanation.
Doesn't really make sense to be honest, so not a good idea. I mean, it worked to a very loose degree in that random fan fiction (because it turns out Wario was only involved in the relationship for the money), but the characters have pretty much nothing in common as far as personality or tastes go.
Now mind you, all shipping jokes I could make aside, I think the easiest answer to this is to go to a sub-section of shipping. OTP, BROTP, and NOTP.
Due to the implications of shipping a child with a smelly italian man, whose sense of morality isn't exactly the best, I think we should avoid the OTP area. BROTP, or friends/bro shipping, is a bit more innocent, and allows us to ship them as friends. NOTP just sort of rejects the idea of them being romantically involved.
I think if you wanted to ship them, do it in a platonic friendly way without any unfortunate implications.
(Plus I mean, clearly Waluigi is the ONLY logical ship for Wario...)