You are correct and I am very dumb. Sorry.
The Clan of Three Swords caters to the needs of fighters including Warriors, Paladins, Rangers, Barbarians, Samurai, and Ninjas.
The League caters to the needs of spellcasters including Wizards, Warlocks, and Bards.
The Guild of the Shadow caters to the needs of all rogue’s including Rogue’s, Barbarians, Bards, and Rangers.
The Brotherhood of Promise caters to the needs of Priests, Paladins, and Warlocks.
So from this I take it that:
Warriors can become Paladins, Rangers, Barbarians, Samurai, and Ninjas.
Wizards can become Warlocks and Bards.
Rogues can become Barbarians, Bards, and Rangers.
Priests can become Paladins and Warlocks.
No, they all can become anything. If you have a warrior, pick paladin, join cleric guild, drop class advancement quest. Or, if you have a cleric, pick paladin, join fighter guild, drop quest, pick Samurai from fighter guild, etc. All characters join all guilds and all can become anything pretty much.
The only outlier in the beginning is the lack of rogue guild in the first town. A work around for that is to pick fighter and advance to Berserker who has rogue skills. Starting class isn't all the important, and you'll want all characters to focus on specific skills.
The biggest thing to look out for is skill changes with new classes and if a new class lets you advance the weapon skill you've been focusing on. It isn't a good idea to spread weapon skills around, or to double dip skills only one character needs (like thief skills and crafting skills). Just make sure you look at the weapon skills of classes you plan on advancing into. Giving everyone sword works well and is good advice for the base game, but that is the genius of Jeff Ludwig's mod, it makes all weapons a good pick and changes some of the skills around and how non-class skill advancement works much better (in my opinion).
If anyone played his Wiz8 mod it really didn't mess with the core game. It made some sensible changes like skills for lord, etc. His mods are done really well and are really thought out with an eye for long term balance and plugging obvious holes and inconsistencies/weird things the core game had.
Edit - traits are important to look at too. For instance, the trait rangers get increase range damage by a lot, so is important to make all characters rangers for that trait you plan on having be ranged combat guys, but wouldn't make sense if you plan on having them use spells while enemies close and then switching to melee. You can never go back to a class, but you can run characters through them just to get specific traits.
Edit 2 - why barbarians are okay to advance into early is because their advancement quest is to kill 20 enemies. Its not a good idea to go fighter to barbarian if you don't plan on grinding some levels since the first dungeon require decent lockpick skill and once you're in you're stuck in for a while. I'm just mentioning incase someone creates a fighter they plan on making a barbarian and no has no rogue and shoots straight into the crypt.
