Can I apply with [Character] from [Season whatever]?
Of course! With the way the plot works, the realities have sort of twisted and bent together, so it's possible to have a character from season one, and another from a later season, of the same fandom! There will be loads of confusion, but that's what makes us unique.
[Character] makes references to [other fandom] canonly, is this okay?
Tread lightly when referencing other fandoms that a character might know about canonly. i.e. Dean's Batman and other Fandom References in character. Feel free to be surprised that such things are actually real, but the major details are foggy. Example: Dean Winchester knows who Batman and Joker are. Pre-Event he knew Batman was Bruce Wayne. Post Event, he's forgotten this spoilerish detail.
[Actor] was on two different shows, can both characters be applied here?
This is addressed in our Face Claim and Shared Face rule. It's perfectly okay. Just imagine Supernatural's Crowley reacting to meeting Firefly's Badger. That would make for some awesome threading!