I think inugamirpg has a good idea when it comes to this. Basically, yes you can have Toothless now, but if one decides to app in later it'll be up to you to talk it out with them, because what will happen is the Toothless you've been playing will disappear and the other player's Toothless won't have any memories of it. Kind of like if a Toothless was dropped and then suddenly apped back in!

I hope this is alright and makes sense.