To answer your first question, you would need to change the Key field for your master table (Record ID is the default). You can do this by going to your table > Settings > Fields. Check the box of the field you want to use as your key, and select "Set Key". Now, when you create the relationship, your tables will be connected based off that field.
For your second question, you can create a Summary Report from your purchase order table. Go to your existing purchase order table, click "Reports and Charts", click "New", then select "Summary Report". Choose the fields you want to summarize under "Summarize Data". From there, scroll down to "Grouping and Crosstabs", group by Date of Purchase, and Combine by Month. This will create a summary report grouped by Month.
I hope this helps!
