Wow, nice story you have here. This might as well be canon.
I mean, we don't know anything about Waluigi's past, and I have always imagined it being a rather sad one. Waluigi being apprenticed by Foreman Spike is something that I can imagine, as Waluigi is constantly trying to prove himself, and goes to far measures to do just that.
We do know a couple of things about Waluigi's past, with the consistent lore being that he spent a long time training hard in order to gain power to match Mario & Luigi so he could attain their glory. The details vary on occasion and I used them to build this headcanon.
We know he spent years training, day and night even, to gain power to beat the Mario Bros, specifically Luigi. We know he trained by "honing his ability to antagonize" as the Melee trophy states, which states he had to learn how to be a villain. The Brawl trophy states he was working hard "in their shadow", and it's canon that he stalks and spies on Luigi to "figure out his weaknesses". It's likely he's been doing it before his debut.
We know his inexplicably petty antagonism of Luigi goes way, way back, before he met Wario or took up his training, and that his rivalry with Luigi is a long term dedication so he can prove himself (one of his appearences in Mario & Sonic even has him state that he thinks beating Luigi will make him the greatest Superstar on the planet). And it's almost entirely one-sided on his end, given how little Luigi cared in his debut, or how Waluigi certainly doesn't feel so strongly about Mario, even though beating Mario is part of why he took up training. So clearly Mario became part of his shitlist later than Luigi for whatever reason.
And we know that he was deliberately keeping a low profile all these years. Recent bios from japanese Nintendo pages point out he was "in a secluded place", and the bio on these images, archived from the Mario Tennis N64 website, even says he was outright hiding. From what, exactly, no one knows.
He also has shown a couple of times that he has some kind of bitter experiences that he doesn't talk about, and that canonically, he isn't as much of a prick to people who aren't Mario & Luigi or don't openly annoy him, like the player character, which just shows how Waluigi isn't really evil at heart despite his bad behavior and self-proclaimed villainy, since all he's after is, besides defeating his rival in competitions, really just recognition and glory.
Additionally, we know that, despite Waluigi's general asshole theatrics and how he and Wario often bicker, we know Waluigi genuinely respects Wario, and some of their interactions and official art suggest that Wario occasionally takes the role of coach to Waluigi (like that official art where Wario is teaching Waluigi proper golf posture). Considering they aren't brothers in established canon, and they couldn't have grown up together since Waluigi doesn't live in Diamond City (although he may not have lived in the Mushroom Kingdom all his life), to me it makes sense to conclude that Wario helped Waluigi in his training. It fits with Charles Martinet's personal headcanon for the two, that they are "two nice evil guys who found each other", and that's how I like to view it.
(he says this after Wario beats him in baseball)
And finally, there's never been any explanation whatsoever as to why so much of Waluigi's character is taken from the Wrecking Crew series, other than the idea that he is a successor to Foreman Spike much like Wario. But to me, it makes sense that Waluigi would purposefully steal and repurpose things from a person he hated into part of his act. Excessive, overdramatic spite is part of what defines Waluigi.
We need Forman Spike to become a recurring character again. He's got potential, and he is a cool guy that I like to see more often.
I just want Spike to come back and bring freaky mechanical dinosaurs with him so he can duel with Waluigi's own robotic dinosaurs.
I would play a whole game around over the top cartoon villains dueling it out with oversized dinosaur toys.
Someone needs to get on this;