I am assuming that you have a custom list and you want customize default list forms...
Every custom list has three pages/forms associated with it:
You can view all these forms in SharePoint designer by clicking the list... and watching the Forms Tab... You can customize these forms to include fields/hide fields from the form, here is a video tutorial on customizing list forms via SharePoint Designer:
And workflows are always associated with list items, they can never be on forms!
And for the second question to allow approve multiple items, you can use this as a solution:
This however does not send just a single item for approval, since all the items need approval all the items will be sent, but to do them quickly approver can select multiple items and approve at once!
Hope this helps...