There should be a way to be able to see the alias you have already created. Since you link (in my case) your simplelogin api. You might reuse some aliases. So there should be away of checking those aliases when you are creating an username. Maybe, instead of showing your aliases in the generator, you could see them in the vault. It could be a type called “Alias” (or aliases), and in case you have your email alias provider linked, it should show you your aliases created.
Maybe you have many aliases provider link to your account. So in order to avoid problems, that “type” in your vault it should just show all your aliases. Maybe it could show it like in folder, like simplelogin/aliases, firefoxrely/alias.
I think it might be really useful even.
That way you don’t need to be jumping between one app and another.
Side note: when you create a new user name using an alias, it should show you a notification or something indicating that the alias was created and you can see it in your alias provider app.