Best way to manage fluctuations in player count and character swapping?

I played solo until I figured out the rules, and then I reset everything. I'm going to start a campaign with 3 players who'll attend regularly (me and 2 friends). But I have some other players who might show up once in a while. I'd like to know, from people who've played further into the game by now, what is the best way to handle this? (I might be missing some information that the game reveals later on, about how this should be done.)

Should the three of us "regulars" switch characters periodically to keep all five character decks at the same relative power level? Because what if we're 20 hours in, and another person wants to join? Won't that person be using a weak beginner deck/character?

How weird would it be if different people play some particular character at different times? Like if we have various guest players who join us off and on, and they all use "Tough Guy" because it just happens to be the only character not being used by the players who attend regularly?

Should new/guest players be run through the tutorial before joining in wherever we've left off in the main campaign?

All tips and advice are appreciated! Try to keep spoilers to a minimum, but if there's rules/mechanics I'm just not aware of... I think I might like to know in advance so I can host gatherings that are as fun for the players as possible.