I know from experience that PLIIx will convert 2.0, 3.0 5.1 to 7.1. John is our resident expert on it but AFAIR it only extracts ambient sounds and sends them to the extra channels. I didn’t catch what type of receiver you have but there should be a selection to have it apply PLIIx to any digital source.
If you only have 4 feet behind your seating I would stick with QS speakers for rears if you choose to go with rear speakers. Just to get some idea of how it will sound if you have a couple old speakers laying around hook them up and place them where you would for 7.1 and have a listen. Won’t be exact but it might give you an idea how much it will help.
Based on your room description I think you might benefit from rear speakers. You don’t have much room behind you for the rear soundstage to develop from the surrounds but do have just enough room for rear speakers to fill in the rear soundstage.
BTW my room is almost the same size 21 x 13.5
