I'm using the av receiver (Marantz NR1604) only with 5.1 channel connected, it's right to connect them in the surround back position or best to use normal surround considering that typically in a 5.1 home cinema they are in the side of the listener?
It depends on what terminals you have connected your speakers to.
normally in a 5.1 set you you have sides In 7.1 (and 6,1) you are adding rears But again it is in the manual for your amp as to which terminals to add the speakers too.
There is likely to be rather more sonic information in the 'sides' than the rears, so I'd be wary of removing that. FWIW, my sides are in the wrong place, being noticeably forward of where they would be ideally. It's not a deal-breaker.
I will try connect to surround 5.1 to get more sonic information and leaving other 2 set of surround back terminal unplugged atm...the speaker setup that I got is: Wharfedale Diamond 10.7, 10.CS, 10.GX, with 10.DFS for surround in the back, in a room of 20m²