i'd recommend having a matched speaker set myself as having multiple model/size speakers might throw off sound a bit. note by matched pair i mean speakers from the same manufacturer in the same series/class. you can use random speakers if you want but if they sound too disimilar you might not be happy in overall sound quality.
as far as going from a 3.5mm to dual binding posts i'm not expert on this but i'd assume there would be some quality loss unless the speaker set uses a 4pole plug. i've never tested it myself though so its only a guesstimate on my part but definitely something you should look up on your own.
5.1 or 7.1 can work in that room. your preference.
not sure on your budget but i'll list what i use...
i'm running a 5.1 audio set in a similar sized room (less length but higher ceilings) and it works out great. preferably i'd have a better placement of speakers but i'm limited due to obstructions. sound delay matched things perfectly though as if it was symetrical so its all good.
i'm running a pioneer vsx-30 receiver with the klipsch quintet IV matched satelite speaker set and the klipsch 450w subwoofer. much better quality than htib sets you can buy and the price isn't as bad as some decent home theater speakers can run.
on the cheaper side of things sony had around 3.5ft tall black tower speakers that used to be on sale at sears.com for $120/pair. my parents have two of them connected unamped to the television and they sound great if a tiny bit muddled.
again, totally your choice on speakers. was just offering up an idea.