Backtagging is totally okay! Canonmates are totally welcome, and cross-canon just as much. Do try not to fourthwall if it interferes with the roleplay.
As for offensive subjects, that's a barrier very difficult to break for me. My boundaries are practically nonexistent, especially for good old-fashioned dark and horror RPs. I'll leave this to your discretion, but don't be afraid of going overboard.
Also, this kid is like, twelve. Use common sense.
If you'd like to avoid spoilers for the movie, please comment below.