I am going to take the advice that many of you wrote: In the introductory one-off ALL players play with new characters, at level 1. I will create some 8-10 pre-made level 1 character sheets and they can grab one (creating 6 new character sheets with newbies takes all day, and I want to play the game, not create char sheets). I will place that one-off in the main campaign-setting but at a geographically different location (i.e. they learn a little about what is wrong with the world, but won't encounter the same locations/NPCs).
The 3 "veterans" indicated that they would like to keep the story going, and also keep their existing characters. And I think I should respect that request. In addition, I spent a fair bit of time to come up with the story so far, and would not mind to keep it going.
So, after getting to know the game with a one-off, we will have a "session 0.1". The three veteran players are allowed to keep their characters (if they still want that), and the others must ensure that their characters will fit in and get along with those existing characters, and also have a motivation to at least start following the main storyline (there is no railroad, so the group can take any random turn at a later stage). Looking at the group, I actually foresee very few problems with this, but I will not skip this session 0.1. At the session 0.1, the newbies will have to create their own character sheets, and we will also roll stats at the table.
Then I will ask the 3 veteran players if they mind if we run the 3 newbies through a little mini-arch before they encounter the rest of the party. Maybe some quick combat against a few kobolds at level 1, and then level up to level 4. If players grumble about sitting out for a moment, then we will level to level 4 immediately.
How many 6 player games have you played?
That is a lot and I think should be your biggest concern. [...]
It's outside the scope of this thread, but you are very much right. I'm both worried and curious to learn if I can handle 6 players. I have experience with 5, but I am aware that one extra player is not insignificant. However, my experience is with 5 rather loud and chaotic players, and this will be 6 more lawful and calm players. (Note that I really mean the players' alignment, not their PCs' alignment). My estimate is that it's going to be fine. But we should probably evaluate this after a few sessions.
Approaching it from the other side: it's one group of friends and it's not easy to kick out just one person out of a group of 6, so I am going to try it. I gave the players a disclaimer already that it's also my first time with 6 players. Again: we should evaluate how it goes.
(also, in the OP, I should have written "
my main worries that I'd like to discuss here are:")