You can do whatever you want as long as you do it well. Unless you are uploading your fanfics to an uptight site that tries to force its values on you, there are no rules, just different opinions. So all I can do is give you mine.
What I can't stand is any kind of written work with gaping plot holes, superficial and/or unrealistic characters, many spelling and grammar errors, and a substandard style of writing. In other words, it's not really about the content of your story, but rather how you write it. If it's a fanfic, I don't like it when the writer completely turns a character around and makes them do/say things they'd never do/say in the original work. However, if you find a good reason for turning Jill evil and Nicholai good, pairing Finn with Excella, or having Ada listen when Leon tells her to wait, go for it. Of course not everyone will like it, but if you actually managed to write something that everybody likes, you would be the first.
As for profanity and violence, well, that also appears in RE itself, especially the latter. Alcohol not so much, but almost everyone drinks it in real life, so it shouldn't be an issue, and I think we can consider it canon that Chris is indeed a smoker. REmake doesn't explicitely show it like the original did, but who would carry around a lighter as their personal item unless they're smoking? Which would technically make Leon a potential smoker as well, in RE2 at least.
But as I said before, do what you like! Just because we have never seen Wesker smoke, doesn't mean he's not a smoker. I've also never seen any of the characters go to the toilet, yet I'm pretty sure they all do that too. You should know for yourself if something that might be considered controversial is actually needed, or if it's just unnecessarily pushing the limits of good taste. For example, if you come up with an elaborate, ridiculously gory death scene, you should ask yourself if that scene is actually relevant in any way (for example to show off how cruel the murderer is), or if the violence is the end rather than just a means to an end. In the latter case, you might consider rewriting it. The same applies to gratuitous sex scenes, unless it's a porn fic (which, by the way, is perfectly fine; feel free to write porn as long as you also label it as such and give it an appropriate age limit on fanfiction.net).
It's just that I feel like I'd be too busy laughing during such a panel to get anything else out of it. DC Douglas cracks me up. His