It's kind of unfortunate that everything has become so political, to the point where people are afraid to RP certain types of people. There's nothing wrong with reaching out, but opinions will range from "There are many different ways you can play this" to "There is only one way to play this, and that is if you have this IRL." It's fine to experiment and play around with an idea even if it's not completely true to life. Everyone has different experiences anyway. Some of my favorite characters do not act anything like real people, but the absurd, surreal like shenanigans they get into are always a joy to experience.
Regardless of what you RP, I believe the best practice when handling something like a disability is to not make it the core of the character. Obviously a story about a character overcoming their disability will focus on the disability itself, but they should have other quirks and interests that are independent of that. As an example, your selectively mute character could be interested in becoming a programmer, or maybe they want to become a stage performer. Maybe their employer activates their tics, or being in crowded spaces like a stage or being nested in hundreds of noisy cubicles. These interests hold up without being selectively mute, but the disability makes them more interesting to pursue. It should be possible to read posts about your character and not be constantly reminded of their disability. It's just part of the character, don't give it any more or less attention unless the situation calls for it. I don't see how that could offend most people.
Additionally, what you don't write is just as important as what you do. If you're going to fuss over the exact sensations people go through with their disabilities, I'd invite you to write their episodes from "outside" the character. I find it rewarding when another player can pick up on hints and correctly or incorrectly has their character react to mine.
I do not have selective mutism, but after skimming some articles it seems like it's tied to anxiety. So you can imagine it being a sort of panic attack that occurs in certain situations, like a selective social anxiety disorder.
Alternatively, and this only works in certain RPs, make your character's disability something fictional. Who can say what sort of quirks your disability has that is similar to but not quite like selective mutism?
With any luck, you won't encounter any
crybullies and will have a fantastic time RPing.
Everyone else gave sound advice. Just be well researched and make sure your group is the sort that would be fine with a character like that, or if it would be problematic with other player's personalities.