Normally Submissives arn't bad ass but its just a sterotype. You should be able to make whatever character you want that looks however they want to look, Ive had overpowered and mean looking characters who are subs behind closed doors.

Regardless of height, muscle mass or otherwise aggressive attitude in day to day dealings, Anybody can be submissive or dom. Big strong muscular guys or girls can be sweet and submissive to their loved ones

and Little tiny thin twigs can be super dom ^^

You can make any character badass if done properly <3

I don't see why not. Just because someone is submissive in romantic or sexual situations doesn't mean they can't be a badass elsewhere. I don't think that being submissive, or dominant, for that matter is something that necessarily has to inform the whole of someone's life. People are complicated and there's nothing wrong with complicated characters.

Honestly, "hard ass/bad ass by night, snuggle kitten by night" is a tried and true trope. By all means, go with what fits you best. I can say from personal experience that there's absolutely a side to myself (and my bun) that doesn't show on these forums. I also happen to know that several of Sinister's more towering and intimidating characters have a secret submissive side they only share with those they trust.

Easily. It's like the idea behind the whole tsundere thing. How you act in public doesn't have to be the same as how you are in private. It's all about your partner and who you're comfortable opening up with.