@Jonathan *******
Your situation is a bit unusual, but everything in Thailand is of course in the discretion of the immigration.
I would just played it safe here and filled out both forms. The requirements for the TM.86 and TM.7 is more or less the same.
Practice can vary, but this is the general requirements for extension of stay;
TM.7
*Passport
*Copy photo page passport
*Copy latest stamp in passport
*Copy visa or extension in passport
*Copy TM.6 if you have. (discontinued for all entries by air, land and sea at date)
*Receipt TM.30
*Lease contract or proof of home
*Pass-photo
*Bank statement or confirmation from your embassy
*1900 thb cash