Well, presumably, your duskull understands that its habit of scaring people and pokémon will need to change, which means it may be easier to work with at this point. Thus, your first step is really to train your duskull proper ways to interact with others. Explain to it that approaching others from the front, fully visible, may help reduce its likelihood of accidentally startling or scaring someone. Have it practice, and reward it every time it approaches you without scaring you so it can understand the right and wrong way to approach someone else.
At the same time, check your baby wooper and work with it to introduce it to your other pokémon. Allow it to get comfortable being around others so that when you reintroduce it to your duskull, it will be a bit more used to interacting with other pokémon.
Then, set up an introduction. Provide your pokémon with a safe, comfortable place with toys and distractions. Get your wooper settled in, then bring your duskull over. Reassure your duskull every step of the way, but have both it and your wooper take things slowly, so to speak. If at any point, they seem uncomfortable with each other, immediately withdraw one or both of them, then try again at a later date.
In other words, the easiest solution to making your duskull feel better is to work with it and to prepare it for a gradual reintroduction to your new family member. If your duskull goes into the interaction understanding how to avoid scaring others and if the reintroduction is slow and comfortable for everyone, then your pokémon should have no trouble becoming friends.
Best of luck, anonymous!