Is it possible to set up the Page Approval workflow so that only people in the Approvers group can be selected in the "Assign To" field?
You can do this by first creating a copy of the Approval - SharePoint 2010 workflow and then open ReviewApproval_1033.xsn Association/Initiation form in InfoPath.
Then right click on the Approvers e-mail adressess -field and select Person/Group Picker Properties. In the SharePoint Server tab write down the address of your site collection and then open the General-tab.
You can select a group from the Choose from dropdown field that you want to limit the workflow to.
Yes, Sure you can set the approval group in the workflow configuration