I finished all 3 of my FF1 runs. Each one flew by....except the Red Mage which took a while. Monk was BY FAR the easiest. Red Mage on the nerfed PS1 version was an extremely slow process. And, get this, four Black Mages in the Pixel Remaster was actually HARDER than the NES version because of Chaos' increased HP. Put it this way, Nuke does about 1.5x the damage it does on the NES while Chaos has 10x the HP. I'd run out of Nuke casts and he'd still be at 25% health. If he cast Curaja or Haste, the fight was over. I was relegating myself to only using NES version items (so no Ethers or Phoenix Downs) because I thought it'd be easier than the NES version. Newp. I finally won by using Haste and a bunch of Tempers on my Masamune-wielder, which made them hit for about 3x the damage of Nuke. So basically the same strategy I used in the Red Mage and Ninja games.
The Monks on the other hand had a really slow start (weaker than Thieves at first), then just absolutely rolled everything in this game from about level 8 onward. Easiest four-character group I've ever played FF1 with, INCLUDING my four Fighters run back in the day.
Red Mage game was just slow. Wasn't that hard, just very very slow.
Overall:
-Monk took the least time and was easiest
-Red Mage took the most time
-Black Mage was the hardest
The PS1 version was just as difficult as the NES version (and with nerfed character level-ups) while the Pixel Remaster had a dramatically harder Chaos, so they likely balanced out.
Finished Monk game with everyone at level 28, Red Mage with the Masamune-wielder at 50 and everyone else at 35, and Black Mage with everyone at 50. Final time was roughly 8 hours for Monk, 13 hours for Black Mage, 17 hours for Red Mage.
I did a 4 Fighters playthrough back in the day on NES and a 4 Ninjas playthrough recently, so all that's left is 4 White Mage at some distant time to give me a reason to do this again. Not for a while though. So far on the difficulty scale, from easiest to hardest, it goes:
1. 4x Monk
2. 4x Fighter
3. 4x Red Mage
4. 4x Thief/Ninja
5. 4x Black Mage
Thief/Ninja is a tough one to rank because Ninjas are pretty good and probably #2 (above Red Mage and maybe Fighter) on the scale, while Thieves are terrible and easily #5 (the worst by far) on the scale. So I averaged it out and put them at #4.
I'm extremely done with FF1 for now.