Playing with people without a season pass

I'm going to try to recruit a couple of my friends to get the game now that it's on sale.

I have a couple of questions:

It doesn't matter which platform they get, iOS, android or steam, I'll be able to play with them, right?

Assuming they just got the base game, would that restrict me to base game stuff when I play with them?  Or could I use Setback while they used base game heroes. What about villains and environments?

Whoever creates the room can select which expansions (that they own) are available. Anyone else who wants to join the room has to own the expansions that the creator selected. So, if your friends only own the base game, you would only be able to use base game heroes/villains/environments when playing with them.