Hey Matthew,
Let me try to answer some of your question from experience.
The 4.x portal uses quite a bit of resources because it is built around a number of docker containers. This makes it very scalable and flexible, but also means it requires much more resources for a small setup.
We typically install all on a single node for the type of use you describe. In my personal opinion, there is no need to have multiple nodes unless it is a very heavily used portal or you require high availability.
If it's only for making the swagger file available, I think you could do that as part of publishing the swagger based service on the gateway. If I remember correctly this is part of the template generated by the swagger wizard. But I don't have an environment available to check this right now. But in that case you would not have the full documentation like the portal would make available.
So I think in your case I would go for a single node deployment of the portal.
Hope that helps.
Michiel