Hi RMatos,
Do you mean that you want to know after an E3 user creates and publish Access Databases, InfoPath forms etc., if an E1 user in the same organization can read/write to these files?
In regard to the issue, if the Office 365 administrator grants the permission the the E1 user, then he can access to the files without issues.
Regarding the second question you’ve asked “Do the Enterprise plans have the Dashboard designer, BCS, WCF, SAP services like on SharePoint server? Can we bring in data from external sources to interact with the databases and workflows in SharePoint online?” , you can develop applications for Microsoft SharePoint Online using Microsoft Business Connectivity Services (BCS). For more details, please refer to: Introduction to Business Connectivity Services in SharePoint Online.
