I don't think of George getting married quickly after Fred's death, though I do agree he would be quite lost. In my personal canon he and Angelina start dating after R/Hr and H/G are already married (though I admit I also imagine those weddings, esp the first, not taking long at all). While the younger couples wait a bit for kids, George and Angelina, being the impulsive type, get pregnant early on in their marriage... perhaps by choice, perhaps not, these things happen. Thus, I have Fred and James as the same age and year at HOgwarts.
It's interesting reading everyone's take on these details, but it's important to remember that, at the end of the day, nearly all of this is just personal choice for the fanfic writer. We are all only speculating... nice to have so much freedom!