I am starting a game that is very melee focused. Are there any big pitfalls people know about that I should watch for or house rule?

Do you mean that your PCs are all melee-focused, or that you are adopting a melee combat focus in general for the campaign? As in, is is that most of your combat will be taking place at very close quarters? Or that your PCs are simply built for that sort of thing?

Most of the weapons in the game will be either melee or lightsabers, with blasters being in sparse use.

You should check out the show Into the Badlands.

Sounds cool!

This being the case, I would make sure to ground any combat scenes very firmly in the narrative. Try to get away, as much as possible, from the "tactical/gamist" play and focus on cinematic combat. This is important for any game, but especially in games where things could get really awkward really fast with everyone being engaged with everyone else...it's important to let there be "something cool" for every PC to do. Maybe one PC is engaged in a brawl with a group of 5 minions, and another PC is locked in a one-on-one duel with a rival, while yet another PC is trying to slice the controls of a door, or download the plans, or use the Force, or set the explosives....etc.

And encourage your players to talk about the scene, and their actions, with narrative flair.

There are a lot of melee weapons that have very low critical ratings and several ranks of Vicious, so it's important to be aware that your nemesis can be killed by a single attack if the player rolls well. You can prevent this by giving your nemesis some ranks in Durable or Center of Being.

I'm also a bit concerned by the reverse as well, but that can be handled a little by controlling the lightsaber upgrades.

Besides all that, I'd keep a close eye on Melee attacks in general. They have, of course, a fixed difficulty, which most and any sort of capable character and NPC should be able to cope with easily. There's no higher difficulty because of the skill of an opponent (other than the Adversary special talent for NPC's ). No opposed roll incorporating the target's Agility or Melee/Brawl skill ranks. just a flat difficulty.

Especially all that with lightsabers, which have all those nice special rules such as Breach or Vicious and whatnot.

Might be a deadly game in general, if people start to forget about Defense scores, defensive stances, Advantage and Disadvantage rules.

Most of the weapons in the game will be either melee or lightsabers, with blasters being in sparse use.

Is this just for the 'on-screen' aspects of the game, or will ranged weapons actually be less common in your galaxy? Will stormtroopers be using vibroweapons instead of blaster rifles, or will the PCs just not generally be encountering enemies that use ranged weapons?

Stormtroopers will use rifles but Stormtroopers themselves will be less common. The planet itself has some strong notions regarding what weapons are proper to use. You can use others, but you will be greatly slowed at gaining influence and allies.

Have plenty of terrain/props/scenery that will give your players a chance to use other skills in combat (namely Athletics and Coordination, but also perhaps Stealth or something else). Otherwise your players may get bored of rolling Melee all the time.

I am now setting it up so they discover the duelist planet 2-4 adventures in and only HAVE to stay there for one if they don't like the hooks.