Hi Eric,
I am currently Beta testing the ArcGIS subscription for organizations and from what my ESRI rep told me in our weekly meeting when I asked that was No, you cannot add geoprocessing services, you need an ArcGIS Server to do that. You will have commercial access to the JavaScript API's, as well as the Flex and Silverlight API's. You can also use the geoprocessing tasks from the sample servers however, there are maximums on the amount of hits using them. For example, if you wanted to use the network geoprocesses such as "find directions to closet facility" the maximum is 25 million (not sure if that is per month or all together) processes. As far as adding a legend, you can use the preconfigured templates in ArcGIS.com or use the API's to create your own. I have yet to beta test ArcMap 10.1 so I'm not sure all the functionality there. However, I do know that you need ArcMap 10.1 to directly upload you maps and make them web maps to your organizations ArcGIS.com subscription.
Hope this helps,
Amber