You must first read Floyd Toole's 'Sound Reproduction' book. Floyd is the authority on speaker placement. Your are already on the right track in that it is the placement that makes the difference.
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Not being able to read but having been into surround sound before most knew it existed; that would be mid '80's--- The room size and all the usual criteria apply.If you like what you have--just buy the speakers--i.e. jump in the water. Rear speakers will add so much enjoyment. There are so many pictures of setups as a starter guide. It doesn't have to be perfect right off.
Heck, starting out with lamp cord for those rears will tell you; as in give you a good taste. I'm sure once you are there in the game you'll know where you want to go.