Clarification on Services & Authentication

[ScottRoberts1]

I'm trying to build a new webapp to serve up demographic, retail and other related geo data backed by the ArcGIS Developer Platform.  I'd like to use the ArcGIS REST services for this, but I have some confusion about which services I could use.  My users would not be known to the ESRI, so I would be using my application account to authenticate.  I'm finding that different services seem to require different authentication and I'm wondering if they are licensed differently.  I'm interested in the plan where I would pay for monthly credits to utilize services.

Could someone tell me if I'm able to access all of the services below using the ArcGIS for Developers platform?

1. GeoEnrichment - https://geoenrich.arcgis.com/arcgis/rest/services/World/geoenrichmentserver/Geoenrichment/Countries/....  This one seems obvious as its referenced under the credit based plans (https://developers.arcgis.com/en/credits/) and I can access it after I authenticate with the token I receive using my app's issued client id and secret from this authentication service https://www.arcgis.com/sharing/oauth2/token.
2. Demographics - http://demographics1.arcgis.com/arcgis/rest/services/USA_Demographics_and_Boundaries/MapServer/.  This one is less clear whether I should be using it or not.  I'm unable to access it using the token retrieved with my app's client id and secret, but it does work if I use my username and password to retrieve a token from this service https://www.arcgis.com/sharing/rest/generatetoken.  It would seem like I should be using my app's credentials to access all services, hence the confusion.

Lastly, there also appears to be this notion of a standard vs basic account and it's not clear to me which one I would need for my purposes (https://developers.arcgis.com/en/licensing/).  I'm not using any of the client SKDs as I'm writing my own http clients to communicate directly to the REST services.  Does this differentiation apply to those who use the credit based platform services?

Thanks in advance.

Scott