Anyone can sign up for Yammer, however you must be invited to join a network. In order to create a network you must have Yammer Enterprise. Yammer Enterprise is available with O365 Business/Education or as a standalone service. Existing O365 Enterprise/Mid-Sized Business/Education plans may be entitled to upgrade to Yammer Enterprise.
If you just want to learn to develop with the Yammer SDK, you technically can get started with your developer account, and just join the Office 365 Community network. However, you would probably benefit from an Enterprise account. Here is more info on Yammer development.
Delve is a BI tool powered by the Office graph. In order to get started using Delve, you must be part of the first release group and activate the required O365 plans and services. Delve and the Office graph are available for O365 Enterprise (E1, E3 and E4), Education (A2, A3 and A4) and Government (G1, G3 and G4). Regardless of the plans you have, the SharePoint Online service must be activated, and users must have a SharePoint Online license assigned. Here is more info on Delve and the Office graph.
Not sure what you mean by 'learn', but a different plan may be required.