HI Vivek
The Fiori UI does not limit what the underlying workflow can do.
Fiori Extensibility relates to adjusting the features of the UI such as which fields are shown or what actions are triggered by custom buttons. Extensibility also does not limit what the workfliw can do.
The number of approvals, type of approvals, identification of workflow recipients are all part of the workflow itself - not the Fiori UI.
Email notification of workflows is typically done using Extended Notifications which is part of the workflow engine. Again this is not affected by the Fiori UI ... Other than of coursw you will want to configure the notification hyperlinks to point to the Fiori My Inbox url
if you need any assistance in adjusting the workflow capabilities please post a quwstion in the SAP Business Workflow space
rgds
Jocelyn