Now that we have the 5 chest run completed, we are going to try to mix things up a bit with Lancers and other class combinations. One thing to note is that a reason you see so many guides with seemingly skewed parties is because while there does exist a margin for error (you can see it in how sloppy we are in this video), the instance is pretty unforgiving if you're trying to go for a "full" completion. The content is still relatively new, so you'll see more and more people stream-line tactics that'll eventually allow for more customization.
One minor caveat, however, is that a balanced party will still probably have 3-4 mages and either a Marauder or a Glad as your main tank. Which means there will really only be 3-4 spots left for other melee class. Another thing to take into account is that that melee class might not play like a "normal" variant of that class. For example, they may be borrowing so many other job skills as to give them a truly niche role that not every Lancer or Pug may be able to fulfill.