1) Have you considered using a portal to Progress located on a Clients layout? This makes it easy to add new Progress records simply by entering data directly into the bottom blank row of the portal. This isn't the only option, but it is the easiest to implement. If you use a layout based directly on Progress, you'll need to implement some method for linking each new progress to the correct client. You can format the ClientID field with a drop down, or you can set up a system where the ClientID number from the current Client Record on a client layout is stored in a global variable or global field so that new Progress records auto-enter this ID to automatically link them to the correct Client record. (This requires first finding the client on the client layout and then clicking a button to perform a script that puts a copy of that client ID into the field/variable and then changes to your Progress Notes layout.
2) This can be problematic if you might have two or more users adding Progress Notes for the same client at the same time. This also may be unecessary. How will you use this number? If it's just to label the sequence of each progress note, the layout text object @@ can be used on the layout to number them in order.
If you do want this number in an actual database field, see this demo file and be very careful how you set up the auto-enter field options or it doesn't work correctly: http://www.4shared.com/file/hB9Oy8r-/SubSerialNumberTest_III.html