Gotchya. Yes, for something brand new, your graph would probably be significantly different from the defaults, but it's still worth taking a close look at the Heritage Resource graph and see how far you can get with renaming nodes... Ultimately, even for making a new resource type, I'd recommend beginning with an existing graph and just adding/removing branches from it.
You should also be aware that these exact modifications are what Arches v4 will have a whole new way of supporting. So, depending on how long term you are thinking, it may be
well worth the wait. There's a development timeline in the
github wiki for arches that shows Farallon's planned development of that version, which ends in May. One of the biggest improvements to that version will be a "configuration manager" which will facilitate the creation of graphs and forms within the app itself.
If you want to begin significant work on the creation of the intangible heritage before then, I'd be happy to help out. The first, and most laborious, step would be to just print out the heritage resource graph, and sit down with the CIDOC CRM, and start modeling branches and nodes... When you have that 95% done (because you'll need to revise it no matter what), I'd recommend that you work with gephi (which can be tricky but does work well) to make the real graph model.
Good luck!
Adam