Philip seemed to miss some of your questions so Ill fill them in:
Mark Yerington wrote:
Can Portal for ArcGIS use the same named user accounts as ArcGIS online or do we need to purchase portal named users
Purchase named users for Portal on Premise. You will receive a specific amount for the level of Enterprise server licensing you get, any additional named users can be purchased in bundle packages.
As an administrator you would log into AGOL (corporate account holder which has access to the licensing) and generate your named user Level 2 and Level 1 perpetual license. In doing so, you are given a Level 1 or Level 2 License number (ECPxxxxxxxxx) to install those counts to your Portal (on premise).
e.g.
Mark Yerington wrote:
It is not advised that you put Portal for ArcGIS on the same server as your ArcGIS server correct?
This depends on your corporate EA strategy. Typically in an enterprise world, no. You keep the resources and server specific to the software. This reduces troubleshooting, patching, upgrading and better resourcing. But as a POC or test box, or simply, can you? Yes.
Mark Yerington wrote:
Also, can portal use the same rest endpoint/ web adaptor that our Current ArcGIS server uses?
You can however that is not a method supported by ESRI. I have done this in the past but would never do so in a production environment.
It is very easy to Re-run the web adaptor install and call the web adaptor something else, you can install many. E.g. arcgis for ArcGIS Server and arcgisportal or Portal for ArcGIS.
You can add additional web adaptors for specific windows authenitcations as well (this example is of course for IIS - windows ) e.g. one for Kerberos, one for NTLM. call it arcgisKerb and arcgisNTLM.
Mark Yerington wrote:
If not we would have to re-route all of our web maps through the new portal correct?
Yep.
this can be easily done in the Portal ITEMS config files, find replace...easy if you know where to look. (no need to create new web maps).
Mark Yerington wrote:
Also can you publish dyanamic map services through Portal of ArcGIS or just feature layers?
the question is wrong since you can publish much more than just feature layers.
What can you add to your portal?—Portal for ArcGIS | ArcGIS Enterprise
Hope this helps
Happy holidays