Home Theatre Speaker Question

I recently purchased a Sony DAV-HDX285 theater system from our local walmart about a month ago, due to my old one getting zapped in a lightning storm. So far this unit has been filling my needs for the most part. I decided on upgrading the speakers on it due to a good deal I got on a set of 7.1 surround Onkyo speakers. I am not very good with audio setup so I want to find out a few things. First, since my receiver is 5.1 channel system and the speakers are 7.1 I would not use the bigger front speakers, or would I replace the smaller front surround speakers with the larger front speakers? Second, I noticed that the theater system has a 4 ohm impedance on it and the speakers have an 8, so would these not work or would I have to modify or wire them differently from normal?