OSR: Multiclassing in the GLOG

The GLOG makes multiclassing ridiculously easy. All classes have 4 templates (A, B, C, and D). You get one per level (1-4) and you have to take them in order. If you want to multiclass, just pick a different template from a different class.

I require my players to justify their multiclassing. There needs to be a good in-character reason, opportunity, and plan. If they even look like they're multiclassing for mechanical optimization, it's not going to happen. 

Weird interactions are adjudicated on a case-by-case basis. Here are a few example rulings.

Bad Cop

Paladin of the Word A, B
Sword Exorcist A, B
Justification: be a paladin, find a sword of exorcism.
The Sword Exorcist class is a ghost detective. The Paladin of the Word commands things with the voice of the Authority, but cannot otherwise speak. Commanding people (or ghosts) to TESTIFY  or CONFESS seems appropriate.

Boomsquad

Many Goblins A,B
Cannoneer A, B
Justification: be many goblins, find a cannon.
Many Goblins got a cannon. I'd rule that, collectively, they can carry it without any issues. They don't get the additional hirelings provided by the Cannoneer A template. Instead, they gain +4 HP, +2 inventory slots, and the ability to carry a cannon without it occupying any inventory slots. Ammo, gunpowder, etc. would still take up some of their 7 slots.

Surly Gnome Massacre Division

Race: Gnome
Many Goblins A
Surly Gnome A, B, C
Justification: this one kind of self-justifies.
Gnomes are small, truculent, and violent. They can turn invisible if they close their eyes, hold their breath, and don't move. The Gnome Massacre Division is a horde of gnomes with sharp daggers and red hats. They drag larger enemies to the ground and pummel them to death with dozens of blows. Basically, you play a nebulous gnome family/caravan/horde. 

Gluttonous Monk

Monk A
Ghoul A
Underground Cannibal Gourmet A, B
Justification: be a monk, eat things (including people) and enjoy it. 
When good monks go bad. You might not do a lot of damage, but the paralyzing claw effect from the Ghoul A template doesn't rely on damage. You are going to write an amazing cookbook and illustrate it too.

Rags to Riches

Barbarian A
Fighter A
Knight A, B
Justification: work your way into the feudal system by prowess at arms, diplomacy, and bribes.
It will take a lot of in-character work to move from an outsider to a trusted member of the Second Estate, but you can do it, and if you do you will become very famous. You will have three attacks per round, deal a ton of damage, and probably start fights you shouldn't start and die because of it. Being good at fighting is dangerous; it means you are more likely to fight. 

Apocalypse Sorcerer

Barbarian A
Sorcerer A, B, C
Justification: be a barbarian, start meddling with things you weren't meant to meddle with. Alternatively, be a sorcerer with poor impulse control.
While raging, the Sorcerer's Harm ability does extra damage. Get mad. Blow stuff up. You can use your other Sorcerer abilities while raging, but only to kill stuff (or make it more likely to die). You can alter things to be dead (they come back after the effect ends), etc. You are even more likely to explode.

Papageno

Goliard A, B
Bell Exorcist A, B
Justification: free-wheeling monk-musician-poet found some magic bells
Very silly, but possibly workable. Heavy use of Kibeth, the Walker, I'd imagine. Make people dance. Charm the birds right off the trees.

Multischool Wizards

Let's say you roll up a Level 1 Elementalist Wizard. Your friend rolls up a Level 1 Garden Wizard. Your friend's character dies 10 minutes into the game from a poisoned doorknob and you loot their spellbook. You like the Garden Wizard spells so much that, at level 2, you ask to level as a Garden Wizard.

1:1 Wizard Schools (Elementalist 1, Garden Wizard 1): Keep the cantrips, mishaps, and dooms of your original school. Gain spells from your new school when you level.

2:1, 3:1, Wizard Schools (Elementalist 1, Garden Wizard 2,3,4): Replace the cantrips, mishaps, and dooms of your original school with those from your new school. Gain spells from your new school when you level.

Basically, you're the kind of wizard you have the most levels in, or the kind of wizard you started as if there's a tie.

Multiclassing After Level 4

You can't.
Maybe you should retire your character to safety and start a new one.