I'm on Chapter 1 and my setup is Ramza as Knight, one male as an Archer, and one as a Monk. One female as Black M. and one as a White M. Seems like a pretty standard setup and I plan on keeping them on their job paths for the most part but I heard some of the end game jobs require you to learn jobs from another path, and even the bard/dancer requires you to be switch magic/physical paths so how am I supposed to go about achieving that?
Here's how it works. Ath the beginning, your path splits in two: Magical(Chemist), and Physical(Squire).
When you follow the physical path, It further expands into two via what I call Strong Classes(Knight, then Monk, then Geomancer) and what I call Nimble Classes(Archer, Thief, Dragoon). If you then mix Strong Classes with a bit of Nimble Classes you get the Samurai(5 monk, 4 Knight, 2 Dragoon). If you mix Nimble Classes with a bit of Strong Classes you get the Ninja(5 thief, 4 archer, 2 geomancer).
The magic path is simpler. It splits into two, Support(White Mage, Mystic, Orator) and Power(Black Mage, Time Mage, Summoner), and going through both gets you the arithmetician(4Summoner, 4Orator).
This leaves the Onion Knight, the Dark Knight, the Dancer, and the Bard.
The Onion Knight is useless. Plain and simple.
The Dark Knight requires you to get lv 8 on the end of both the Strong and Nimble paths, plus master the Knight and Black Mage.
The Dancer is a Female-only job. It requires them to go through the Physical jobs(5 Dragoon, 5 Geomancer).
The Bard is a Male-only job. It requires them to go through the Magical jobs(5 Orator, 5 Summoner).
The only fancy jobs worth bothering with are Samurai and Ninja. You'll need to invest in both "arms" of the physical job chains:
Heavy Knight/Archer, Level 2 Dragoon --> Samurai Heavy Monk/Thief, Level 2 Geomancer --> Ninja
Note that these jobs aren't worth it until their weapons become available, which won't happen until late Chapter 2, maybe early Chapter 3. The other fancy jobs really aren't worth it unless you're just looking for a novelty playthrough.
Female gets 4 Dragoon/Geomancer --> Dancer Male gets 4 Mediator/Summoner --> Bard Heavy on both magic halves, any sex --> Arithmetician Grindlikehell --> Dark Knight
As a side-note, you're probably better off long-term investing in a Chemist as a healer instead of a mage. The reason for this is that healing magic is really underpowered in this game -- their abilities are slow, require MP, and are inefficient on units with low faith. Chemists don't have any of these problems. Additionally, Chemists get Auto-Potion and Throw-Item, which travel very well to any other job, but the White Mage only really gets White Magic, which only works well on the magic jobs.
Black Mage isn't not bad, but you'll probably want to get Time or Summon magic with it (both of which are part of the Black Mage chain).
Just to make a correction about what someone else said, Onion Knight is not useless. Well, it's useless at the start. How it works is after you master all 14 jobs except Squire, Chemist, Dark Knight, and Mime (lvl 8 Squire, lvl 8 Chemist, lvl 5 Geomancer, lvl 5 Dragoon, lvl 5 Summoner, and lvl 5 Orator). Once you master all those 14 jobs you unlock what is called True Onion Knight. This version of Onion Knight has the best stat growth in the game and can equip the onion equipment which is the best equipment in the game.
The downside is they can't have any abilities and are basically powerhouses. They also can't gain exp, but can gain levels through a trick using Beastmaster support ability with Wild Boar monsters. You have to breed them.
On a side note since no one mentioned Mime, they have some of the best stat growth in the game. The only bad stat is MP. They are a bit tricky to use, but can make for some interesting fights.