To be honest, when I see something like Kanda X OC, I get the same vibes as when I picture Kanda X (girl) Alma. So...yeah. Lavi, possible, though in his case the girl would likely die and Lavi would become more Bookman-ish. And I could imagine him pretending to be a girl's lover in order to get information or just to blend in or something, back when he was not so nice. Allen...no. It just doesn't feel right to me. For Kanda, I can imagine it at any time, Lavi, definitely some time in the past, and Allen...I feel like it would be impossible in the past, and him meeting a girl he likes now is rather unlikely...though maybe if he was hiding from the Order by himself, he could meet a villager/farmer/peasant girl (who might be the one hiding him in her house or something) and they could maybe get a thing going, but in the end he'd go back to the Order and do epic battles and blablabla.
Those are the weird impressions I guess.
EDIT:
http://shiikane-chan.deviantart.com/gallery/30981461#/d3jp8ok
:'D There are so many faults in this pic I can't even name them all.