I do not believe there are any specific guidelines. (What 'additional sheets' do you mean anyway?)
That said: if these are docs you are scanning into PDF, surely it will take minimal additional effort to sign them before scanning (or paste signature in an electronic version). Outside docs - scans of receipts etc - don't need them unless they are things that would clearly have to be signed.
I didn't sign any additional documents aside the forms, I've submitted additional documents for family info, proof of relationship, and so on.
They were all accepted.
Yes 100%, I feel like if you stick with signing any oficial form and any letter (support, explanation, etc) you should be good. The main thing is the forms, as those are mandatory to even start analyzing your process. Anything else they are able to request later if there is something missing.