I think it should be allowed, as, like Gooz pointed out, Sam is not a member of Vanguard as of now, and the purpose of the story is to show who will stay and who will go away.

That also means, obviously, that one day you will be allowed to have both Axel and Mason in a story together, if and when you will decide to switch their status.

And I did the same in issue 12, where I had both Nadia and Euro, being Nadia a guest star in that moment.

About Leslie and Phil, I have no problem in having them used (one at a time) as main characters for Sammitch in different stories, but still I would prefer to see stated that one is an official member, and one just an associated employed, like Grissom is. But that's just to show that the line-up has just one character per writer, just that.