I’m not sure if I understand correctly about the project you are hosting.
The basic norm is if your usage is under the Hobby plan quota and you’re not charging customer to use your service hosted on Vercel. You can safely continue on the Hobby plan.
Oh, I see. I haven’t made a SOAP service myself but assuming it’s built on a supported runtime, you could potentially have it in the api/ directory in the same project.
By the way, I’m curious why don’t you want it on a different project?