A lot of questions there...
 
Notes - First off, it depends whether you have Evo Notes turned on or not. I do, and think you should, so I'll answer accordingly.
 
The advantage and disadvantage of the EN system is that it is extremely flexible. That means the ways of using it are almost endless. I strongly advise spending a lot of time figuring out how it works, then setting up a protocol that all users must follow when using it. Otherwise someone will enter an important note, and the person that needs to see it, won't.
 
Hidden notes are not really applicable with the EN system. Yes, you can use them, and the designation is meaningful, but as far as functionality, they are no different than any other kind of "pre-packaged" or user defined notes. Whether a Note Type shows up on a document or not is determined on the Note Selection dialog that pops up every time you print documents (unless you have Quick Printing turned on, which I highly recommend once you get all the EN options figured out).
 
We have a matrix that shows what note types are to be used where, and what documents they print on to help people know which type to pick. I'd be happy to share that with you if you message me your email address.
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I don't believe you can add notes to a SO once it's closed. However, depending on your settings, you can re-open a closed SO, add notes, and then close again.
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The file name for forms editing will depend on how you're using Evo. Again, assuming you have EN system you will be using forms with this format:
 
ENSOCX.RTM
 
EN - always the same
SOC - Menu name/letter of printing program, i.e. SO-C
X - the number you select in SM-D
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All the best,

We do use HID notes for things we don't wan the customer to see. Basically, the only print on the SO Pick List.
 
We also have an INT (Internal) type for information we don't want to print ANYwhere - most if not all these have alerts set so that you look at them when entering an order.
 
Then, we have a FIN (Finance) type for information regarding collection calls. We print those on the aging report so we can see if they followed through with what they promised...
 
We also have a MFG (manufacturing) type. We use it in two ways - if there is a Line Item note on a SO for an item that is going to be made into a WO we use that to make sure the note follows the WO*. But we also use it on Items where we want the note on the WO, but NOT the Sales Order related docs...
 
So, you see, just having a Hidden note is helpful, but you can go a lot further.
 
 
*You can't just use the STD (Standard) note type, because if you print Travelers with SO STD notes you will get not only the SO Line Item note, but also any general SO note. This can be very confusing to manufacturing - like "Please email Johnny when order is ready to ship"...