And that is the entire point of this discussion. Some preference do not have a mutual compromise. If I am going to run ToTM onle, and someone else wants tactical combat, that's not something that can be compromised. If a table wants to run ONLY classic archetypes and NO multiclassing, and a new player wants to run the coolest new UA thing, that's not something that is reasonably compromised either.
Yeah, compromise should always be achieved, and that's why we communicate; but sometimes compromise means someone has to give up their fun for the greater good. And maybe it means that the player killer becomes a team player for the campaign, and everyone agrees to play the occasional one-shot of Paranoia.
Whoa, you're conflating very different things here!
TotM versus grid: some game rules are toggled 'on' or 'off', and everyone in the same game must use the same rule. For example, while playing on a grid, some tables might run diagonal movement to take an extra square (5ft) for each even diagonal (like 3e), and some tables use 1-for-1 movement, diagonal or not (like 4e). In such a case, it doesn't really matter which the table uses, but it MUST be that ALL creatures, PC or otherwise, use the same rule.
Multiclassing: IF the optional rule is allowed in this campaign, but the players are playing single class PCs, then a new player introducing a multiclass PC in no way spoils the other players' fun! They still get to play their own PC in any way that want.
The same applies to fluff. Let's say that one player (or DM!) treats character classes as inviolable archetypes; that 'barbarians' are one thing in both game mechanics and in world culture, etc. for every class. Let's say another player (or DM!) uses the game rules to make a RAW PC but uses their own fluff for their own PC, and the game mechanic of 'class' is a metagame construct that has no existence in the game world. Can these players play at the table without either destroying the other's fun by their very presence? Of course!
Taking my werewolf-inspired barbarian. In world, my PC doesn't approach the other PCs and say, "Hi! I'm a barbarian, but weirdly I'n not really barbaric, culturally speaking. I'm a special snowflake!"
No, my PC introduces himself to the PCs (and anyone else) in game by saying, "Hi, I'm Captain Finn Winter of the Avant Guard!". Finn would
never think of himself or describe himself as a 'barbarian'. Such a thought would never enter his head! It would be absurd, because our PCs cannot look at their own character sheets! They don't realise that they are made-up avatars for 'real people' to have a bit of fun with their mates once a week (if we're lucky!); they have no knowledge of their own 5e rules 'class'.
So, the other party members might want to know what I can do, what I can contribute to the team. Sure, me
the player could say that I'm a Bar 3/War 6, focusing on getting the most out of
armour of Agathys and Damage Resistance, but my
character could never say such a thing because he cannot be aware of the metagame.
It would be like a comic superhero being aware that he is a fictional character. When Deadpool does this (with the superpower 'Comic Awareness'!) it just illustrates that this is something that fictional characters
cannot (usually) do.
So, when asked, Finn would say that he is good in hand-to-hand combat, and his military training was that of pathfinder/scout/commando-type stuff, but he
cannot say that he is a 'barbarian'! The only place that word is mentioned is on the character sheet; it doesn't exist for him in the game world.
So what PCs can know about each other is equal for every PC; "I'm good at (x and y)". They
cannot know if the others are 'single classed' or 'multiclassed', because that is metagame knowledge.
Given that, each PC is an individual. (I'm Not!
Shut up!) Every PC is their own 'special snowflake' in that sense. Therefore one player's PC cannot spoil the 'fun' of the other players merely on the basis that
this PC 'changes the whole world'! They also cannot complain about the metagame, because multiclassing IS allowed in this campaign, but they CHOSE to be single class themselves.
Player 1: I'm playing a wizard. What about you?
Player 2: Cleric.
Player 1: Cool! What about you?
Player 3: Rogue.
Player 1: Cool! You?
Player 4: Fighter.
Player 1: Cool! What about you?
Player 5: Barbarian/Druid.
Player 1: How DARE you spoil MY fun!
Player 5: ...what...?
Player 1: You have to play a single class PC, because if you don't then I won't be able to enjoy myself!
Player 5: ...but the DM said that multiclassing is allowed...
Player 1: It's a well known fact that each player gets to veto each other player's PC!
Player 5: Okay, you can't play a wizard. It will prevent me from having fun.
Player 1: How DARE you tell me what character I can and cannot play!